As I read it the CAA require an exemption application for this purpose but are happy to provide. As for the practicalities it seems you went about it in a well organised manner only to be caught out by unexpected 'dampness'. I have seen this done from a Tiger Moth and there is still some evidence of the remains in the furthest recesses of the cockpit. In that case it as simply leaning over the side and emptying the urn. The trick to avoid too much 'blowback' was to cross controls so that there was no airflow into the 'drop' side.