We've found Nationwide to be eminently fair about such things. We had been fretting about our fixed rate deal coming to an end last year, believing that the SVR would be very expensive and bemoaning the fact that fixed rate deals had become much more expensive. We were bracing ourselves for our monthly payments going up by £130 when everything went wobbly. In the meantime we had applied and been approved twice for fixed rate deals that had a reservation fee of £299, but when our existing fix ended we took advantage of the massive drop in the SVR. What we didn't realise until we got our annual statement the other week was that they'd added the two reservation fees to our outstanding mortgage balance, even though we'd not in the end signed up to the fixed rate offers. But when I phoned them, they were very co-operative and immediately acknowledged their error and arranged for the fees to be refunded.
So persist - I find it hard to believe that they'd have used any tricksy small-print to force you onto a fixed rate on expiry of your existing deal - their bumph has, as long as I can remember, always said that the rate reverts to the SVR at the end of your fixed period, not onto some other product of their choosing.