Egg / Barclaycard PPI redress

Hello,
First post here but I've used the site for a while and found the PPI advice very helpful.
This year I decided to claim for mis-sold PPI from my Egg (now Barclaycard) credit card on the basis that I would not have received correct advice and online boxes were pre-ticked when I applied.
I first took the card out in October 1999 and it was replaced with a Barclaycard when they took over the Egg Card business.
My complaint was declined by Barclaycard, so I escalated it to the financial ombudsman. They were very helpful indeed and after an email conversation with them through the Resolver site they discerned that I was mis-sold PPI by Egg due to me not having been working full time in permanent employment in the 6 months prior to my initial card application.
I am pleased to say that Barclaycard have now made an offer as follows, after cancelling the PPI policy last month:
A - Refund of payments made by you for PPI cover - £3206.69
B - Refund of interest charged on PPI premiums - £1275.70
C - 8% interest statutory compensation - £2785.62
Less income tax deduction - £557.12
D - Total payable - £6710.89

Now my query is with item B, the refund of interest. Does this figure seem right for 18 years of card interest on £3206.69? I am not sure how they would have arrived at this figure and was wondering if it was possible to verify the figure at all?

Has anyone else had a refund offer from Barclaycard and had to query the offer in the same way?

On another note their offer letter says that the ombudman says that I have already accepted the offer, which I have not.

Thanks in advance,
Andy