Jetstream Rider,
If the IR decide that they have made a mistake and shouldn't have given you tax relief on BALPA subscriptions then guess who will have to pay back all that tax relief? You! BALPA subscriptions are not tax deductible. I would tell them before you get yourself into a situation where in a few years they notice their mistake and send you a bill for a hundreds of pounds. It's awful when that happens - I know - they just sent me a bill for almost £2000 because someone keyed in the wrong amount for private medical insurance many years back (and a few other small mistakes that all added up).
You shouldn't need to worry about keeping receipts for your £540 allowance. Receipts are only necessary if you have spent, and claim for expenses in addition to that amount. The £540 is available to everyone regardless of whether you spend it or not.
Hope this helps.
Lazlo