Dave, you may find that the initial letter about the assessment warned that if you "failed to attend" your benefit would be stopped. The DWP may argue they notified you therefore of the penalty well in advance.

However here are the sections from the "Decision Makers Guide"

This product is designed for staff who make decisions about benefit entitlement. Volume 1, Chapter 2, Paragraphs 02372 – 02377 may be of particular interest to you, as it contains details of the circumstances under which backdating may be allowed.

Extending the time for claiming IS and JSA

02372    

The time for claiming IS or JSA may be extended by a period of up to

1. one month (see DMG 02374) or

2. three months (see DMG 02376)

if certain conditions are satisfied¹. But the periods cannot be added together².
 

¹ reg 19(4), (5), (6) & (7), ² R(IS) 3/01

02373

In both cases the period of extension is that falling immediately before the date of
claim. It does not necessarily run from the first day of the period claimed for¹. In
appropriate circumstances a claim can be taken as including a claim for a period
starting with the earliest date which would make the claim in time².
 

¹ R(IS) 3/01; ² R(IS) 16/04

One month extension

02374

The conditions are that

1.     the claim is made late and

2.     one or more of the circumstances specified in DMG 02375 apply and

3.     as a result of those circumstances the claimant could not reasonably have
been expected to make the claim earlier¹.

¹ SS (C&P) Regs, reg 19(6), R(IS) 16/04

02375

The circumstances are¹

1.     the office where the claimant would be expected to make a claim was closed
and alternative arrangements had not been made

 
2.     the claimant was unable to attend the appropriate office due to difficulties with
the normal mode of transport and there was no reasonable alternative
available

3.     there were adverse postal conditions

4.     the claimant was previously in receipt of another benefit, and notification of
expiry of entitlement to that benefit was not sent to the claimant before the
date on which entitlement expired

5.     in a claim for IS or JSA the claimant had ceased to be a member of a couple
within the period of one month before the claim was made

6.     during the period of one month before the claim was made, a close relative of
the claimant died. “Close relative” means a partner, parent, son, daughter,
brother or sister

7.     a claim is made by one member of a joint-claim couple and the other member
has failed to attend at a time and place specified by the Secretary of State for
the purpose of claiming a joint-claim JSA

8.     the claimant’s partner was previously in receipt of another benefit, and
notification of expiry of entitlement to that benefit was not sent to the
claimant’s partner before the date on which entitlement expired

9.     the claimant was unable to make telephone contact with the appropriate office
because the telephone lines were busy or inoperative.

¹ SS (C&P) Regs, reg 19(7)

Three months extension

02376

The conditions are that

1.     the claim is made late and

2.     one or more of the circumstances specified in DMG 02377 apply and

3.     as a result of those circumstances the claimant could not reasonably be
expected to make the claim earlier¹.

¹ SS (C&P) Regs, reg 19(4), R(IS) 16/04

02377

The circumstances are¹

1.     the claimant has difficulty communicating because of

1.1     learning, language, or literacy difficulties or

1.2      deafness or blindness and it was not reasonably practicable for the claimant to obtain assistance
from another person to make the claim

2.      except in the case of a claim for JSA, the claimant was ill or disabled, and it
was not reasonably practicable for the claimant to obtain assistance from
another person to make the claim

3.      the claimant was caring for a person who was ill or disabled, and it was not
reasonably practicable to get help from another person to make the claim

4.      the claimant was given information by an officer of the DWP which led the
claimant to believe that a claim for benefit would not succeed²

5.      the claimant was given written advice by a solicitor or other professional
adviser, a medical practitioner, an LA, or a person working in a Citizens
Advice Bureau or a similar advice agency, which led the claimant to believe
that a claim for benefit would not succeed

6.      the claimant or partner was given written information about income or capital
by an employer or former employer, or by a bank or building society, which
led the claimant to believe that a claim for benefit would not succeed  

7.      the claimant was required to deal with a domestic emergency and it was not
reasonably practicable to obtain assistance from another person to make the
claim

8.      the claimant was prevented by adverse weather conditions from attending the
appropriate office.

¹ reg 19(5); ² R(IS) 3/01

Example 1

 
A person contacts the Jobcentre on 18 January with the intention of making a claim
for JSA. He is given an appointment on 20 January. At the interview he says that he
last worked on 5 January and wished to claim from 6 January. He says that he has
not made a claim sooner because he had contacted the secretary of his trade union
who advised him in writing that he should not register as unemployed or claim
benefit until the reasons for his dismissal had been fully investigated. The DM
decides that he has been given written advice by an adviser which led him to
believe that a claim for JSA would not succeed and extends the time for claiming
from 6 January to 18 January.

Example 2  

A person in receipt of JSA falls sick on 25 January. He contacts the Jobcentre on
that day and is advised to claim IB, but is not told anything more. He is informed on
3 February that he does not satisfy the contribution conditions for IB and he then
makes a claim for IS. He says that he was under the impression that IB was all he
could claim and did not realize that he could claim IS as well as IB.  
The DM decides that the person has been given information by an officer of the
DWP which led him to believe that a claim for benefit would not succeed and
extends the time for claiming IS from 25 January to 3 February.