I have not heard of an Upton camp but there was one on Bidston, down School Lane and, of course, RAF West Kirby actually between Greasby & Moreton, also not far from Saughall Massie and Upton, could have been the same place.
I hope that I am not 2 years too late with this information but this site (http://www.upton-wirral.co.uk/upt68.php) has information about an army camp in Upton which may well be what your grandfather was talking about.