Fantastic! Thanks for the response, I thought I was going to have to dig out about 18 inches which was putting me off the more I thought about it.
Another thought..the gravel that's around where the pond was originally has a depth of about 4-5 inches which can be dug out easily, after that not so sure. Will it be ok to do the same with the topsoil over this?
Should be no problem. Grass will grow anywhere. Its called "hard standing". When you think of all the cars, caravans, cycles, people, etc, that walk about on camp sites, well, it says it all, you don't come in covered in mud. The only thing is that may be, if you get a very hot dry spell, you may need to water it, but it doesn't matter, once grass takes root, it will grow back, even if it turns yellow.