Ok, the problem has been solved!
It turns out that the replacement valves, having been on there for over 10 years, had got worn down due to regular pressure checking and re-inflating when needed.
This had caused the threads to become a little narrower such that the attachment on the hose from the compressor wouldn't actually engage with them.
A quick trip to Russ' Tyres in Portsmouth for a new set of valves and now everything is working fine
Thanks for the comments and advice everyone