I have an electric (battery) ryobi pole chainsaw, it's great. I sometimes use it with an additional extension pole as I also got a ryobi pole hedge trimmer and the extension poles are the same. However I'm over 6 ft and 80 kg and at least averagely strong for a 56-yo man, if you're an average female build you may find it a bit of a struggle. It's quite a workout.

If you're on good terms with the neighbours it's probably easier for you and better for the plant to go round to their garden and prune it properly rather than trying to use anything on a pole from your side. Pole mounted pruning saws can be hard work as you obviously can't hold the branches you're trying to cut and they will bounce up and down as you try to use the saw. If you're using pole mounted loppers it's worth looking for a set with some sort of pulley to reduce effort on larger cuts, and better if one person manages the pole and someone else does the cutting mechanism. Or as the saying goes "if a job's worth doing it's worth buying a power tool" look for a small pole mounted chainsaw and appropriate PPE (chainsaw accidenta are rarely minor).

As a much much lighter alternative, my parents have (inherited) a fantastic mechanical pole cutter which is amazingly effective for anything up to a couple of inches and light enough for my mother to use (at least until recently) due to being a thin-walled aluminium pole. But I've never seen anything as good as it in shops, modern mechanical models aren't a patch on it (though they still may be your best bet).

Very helpful posts, thank you.  Anything with a battery will be heavy and don't think I could manage that.  The lopper I'd seen has a pulley system.  Have a tricky situation where the owner of the garden has dementia with carers visiting regularly.  Last thing I want to do is to cause any distress or disturbance.  From their garden it's not possible to see the overhang and in any case their garden is to the side of their house and not visible from the house.  Thought that a lopper or something could be an interim measure until such point as the situation changes.

Thanks again.

The right sort of lopper can cut through some quite big branches. I inherited a wooden one that has a blade at the top that is operated by a lever and a metal rod, I also have experience of the pulley type, these often have a saw blade that you can attach for larger branches. Lidl and Aldi do them sometimes and B and Q/Screwfix plus some garden centres. 
My tip is to make sure that all the fixings are as tight as possible to minimise free play and also keep the blade as sharp as possible.

These from Fiskars

PowerGear X Telescopic Tree Pruner FSK1023624

should be good, if pricey. Their other garden tools are excellent. Screwfix have the house brand, Magnussun ones on offer and cheaper than B n Q, probably heavier than the Fiskars though.

Hope this helps.

Thank you and am liking that one, it's made of aluminium so weight shouldn't be an issue.  Definitely a contender.

What is the greenery and size of it you are looking to cut?

High branches on an oak would probably be a pole saw (silky) or power pruner as they are chunky and stable.

A hazel or Rowan maybe one of the extended loppers as they move about under a saw.

Beech or something fine and droopy I've found easier with a hedge cutter attachment on a power pruner. 

All done stood on a Land Rover roof to extend reach.

Cut 10-15cm out from the growth point with a pole saw or whatever cutter you get. Don't try and get a clean cut the first time unless you have decent kit. Once you have removed the weight you can do a clean cut.

I've got a Fiskars UP86, which is the one before the PowerGear^^

Easy to use but be careful about cutting directly overhead. You only do it once before looking at head/face protection 🙂

The advantage of the lopper is that you tend to get a clean cut (see also mrl^^) and they're quiet and lighter too

Apparently it's a sycamore and don't believe it's the same age as the other sycamore trees in very close proximity, they are huge.  At this stage it's only what's overhanging in my garden, some narrow-looking branches will be removed.  In an ideal world I'd like the height reduced considerably of this particular sycamore but will wait until the situation changes with regard to the current occupant.  Good point regarding cutting directly overhead, thank you!

Appreciate the responses.

I have the Fiskars loppers and the saw attachment, they're very good.  I'm a gardener so they got a fair bit of use.  As long you don't try and cut anything too thick with the loppers you'll be fine.  Take your time and work your way along so you're not cutting off too much at once.  The saw is quite hard work at anything like full extension but it is pretty effective.

Just to update ... went for the Fiskars PowerGear X Telescopic Tree Pruner and it proved to be a decent bit of kit.  Extend from 2 metres to 6 metres which is handy.  Whilst I didn't use them personally my 'gardener' made quick work and I cut them down for the garden waste bin which is now full.  Has made quite a difference to the garden, thanks again for the recommendations.