The man at WGC was a bit upset about having to do it, but did look up the info for me as I helped him find the ticket by giving him the price. I'm sure he found the ticket that way. Then off he went with calculator in hand, then came back to say it was £240 something - but the ticket actually shows £504.70.
A bit late now, as you've already got the ticket, but here's how I'd work it out:
Original ticket for Hatfield - Hadley Wood, valid from 04 JNR 2010 to 03 JNR 2011.
Price of original ticket: £1016.00
Base rate for original ticket: £1016.00 / 365 days = £2.78 (to nearest 1p)
Cost of Hatfield - Zones 4-6 Annual Travelcard from 04 JNR 2010 to 03 JNR 2011: £47.00 * 40 weeks = £1880.00
Base rate for new ticket: £1880.00 / 365 days = £5.15 (to nearest 1p)
Starting date for new ticket: 28 SEP 2010, Valid until: 03 JNR 2011, Number of days Validity: 98
Credit due on original ticket: £2.78 * 98 = £272.44
Charge for new ticket: £5.15 * 98 = £504.70
Payment due from customer: £504.70 - £272.44 = £232.26
He had to do a manual payment too, so he took my word for it that the card would authorise (it will) and gave me an old-school slip of paper with an imprint of the card. Haven't seen one of those for a while.
I'm not sure what Ticket Issuing System the GN side stations are using these days, but what he
should have done is issued the ticket at £504.70 (which he did), along with a Season Ticket Refund Sundry for the £272.44 credit, then processed your card through the TIS in the normal manner for the £232.26.
If he didn't know how to do that, the next best option is to put the £272.44 through as some other method of payment and charge the balance to your card.
If that wasn't an option for some reason, he should've phoned up the Merchant Services Provider for an authorisation code and written that on the manual voucher. (Though having said that, the industry has recently changed providers, and if it's anything like our place, they won't have updated the imprinter plate and fallback vouchers).
HTH,
Barry