I know what you mean about it being costly to find a new food.
I would try Pets at Home and get a few single sachets of some affordable food just to try out.
My two used to like the hi life potty about gravy - I went into our local (very small) pet supplies shop and asked for it once, I think thy thought I was the potty one!
Now they love Felix sauce sensations, but what one cat loves, another will no doubt turn it's nose up at!!!
Have you tried heating the food in the microwave just for 10 seconds? I know it's warm at the minute, but having it heated night make it more tempting?!