Gary,
It looks as tho' I have a similar problem - My 1968 Three Thousand Five exhibits all the symptoms of a flat battery but the battery appears fully charged!
Checking my receipts tells me that this battery was bought in April 1999 (almost 5 years old) with a three year guarantee for £45.25.
Assuming that your battery fits in the boot, the physical size is important as the first one bought (last time) looked right but would not fit in the battery box!
Some part numbers for you - Unipart GBY 071 - Lucas 071 - Celtic 369/069 - B.I.G. 071 55031 - The last two may be local suppliers/manufacturers. Hope these help.
Let me know how you get on and I shall report back on my success, or otherwise!
Regards, John.