Wow. This forum really is full of amazing people! I'm going to go with the 25's. I've been on some pretty bad surfaces with them and they have always been OK. Polish gravel tracks may be different tho.
Thanks very much for the offer though, you legend!
Are the 25's unused? I'll need to but a pair of 25's at some point, so if you do want to order some 28's, I'll take them off you.