Wow! Lovely big old tree, Ladyessex! They usually don't need pruning, but if you are going for size reduction, it is best to get a knowledgable professional to do it. Is there the equivalent of "certified arborist" in the UK? Above all, don't let anyone who wants to chop off all of the branches touch it!
I personally would prune it in early spring--after hard frost, but before sprouting, but the folks from the UK on this site may have a better idea of timing.

The tree is overhanging neighbour's gardens, it had a load of fruits this year but they didnt swell & are just falling off, this is the 3rd year it hasnt done well, the best time of year I like it is in the late autumn when all the leaves have dropped off it looks awesome against the sky, the birds find it useful to perch in it too, I would love to have some expert prune it but that is a cost I could do without at the moment so my have to put on our long gloves & do it ourselves, it needs opening up as lots of crossed branches plus all the suckers coming up around the bottom, I will wait till early spring as you advised :o))