An Anytime Day Single Colchester- London Terminals is £27.70, so I've no idea where the £57.40 comes from.
It sounds like the OP is in a difficult position.
On the one hand, the PF is 100% invalid and not due, but, on the other hand, if it is not paid, he/she will probably be prosecuted under byelaw 18, and end up paying far more.
The Penalty Fares Rules state:
7.6 An authorised collector must not charge a penalty fare to a person whose ticket is not valid only because of a published restriction, as described in condition 12 of the National Rail Conditions of Carriage.
There is nothing in the PF Rules providing that the above rule does not apply simply because the ticket cannot be excessed.
It wouldn't be the first time a TOC has incorrectly charged a PF to passengers for travelling on an otherwise valid time or or route-restricted ticket, knowing that there is no truly independent appeals system (IPFAS/IRCAS are not fully independent), and knowing that the passenger is 'over the barrel' and left with little choice but to pay the ransom demanded.
I agree that contacting London Travelwatch would be a good start, however.