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I know I probably haven't done it right so far but I have a field maple in the garden that I've kept cut back to prevent it hitting the overhead wires, and it needs doing again.

Can anyone point me in the direction of proper instructions for pollarding?

I think I should have trimmed it down last month but failed due to my being not-quite-tall-enough to reach, even with long handled loppers!
When I first took the top out I forgot that the trunk would grow as well as the branches

Based on nothing but instinct I'd be thinking that a maple would probably bleed like crazy, I'd be looking into whether there is a "right" time to do it - we have over 30 walnuts and the received wisdom is to only prune in the (first) new moon of august, well first year my (tree surgeon) ignored this and we had sap coming out of them for days, last year we did it as suggested and no problems (except the one we later realised we'd missed and went back to as it was interferring with our solar panels). Anyway gone a bit off on one but just wanted to say that IME there is something in it and maples strike me as being very sappy trees too.

pollarding is the same as coppicing, but done higher up the tree. Generally the best time to do it is the winter, during the trees dormant period.