A couple of links that may help with the validity issue and keep your chart right up to date.
CAA DAP Aeronautical Charts & Data Section maintain a website of past and future amendments to both....
Quarter Mil Series
and
Half Mil Series
Giving the pilot the opportunity to update their own chart and reduce the necessity to purchase a new chart more frequently.
Also try this supplier for the paper chart variant
Documedia
Failing that, here's a list of stockists.
CAA VFR Chart Stockists
Failing that try e-mailing the section at the address below:
Aeronautical Charts & Data Section
They are very open to ideas
David, try them and they may be able to help directly with your requirements.