TMH wrote:

Anyone used one of these telescopic ladders?

I've used one provided by a school to reach the double height ceiling in
their hall, I'm no lightweight and they didn't feel flimsy.

The main use will be cleaning/repairing gutters

I just use my two section extension ladder with a stand-off to reach
guttering.

don't need to get on the roof. I have a suitable double extension
already, but sometimes getting it through the house to the back is a
problem.
Ah, understood; I can take mine out the front or rear of the garage.

So, what height do I need? They seem to only go up to 5 metres.

You should get away with a little less than that, depending on height of
eaves (2 x 2.4m ?) and your own height, e.g assuming you want to stand a
few rungs down so your shoulders are just above gutter height, and
allowing for the angle of the ladder.

ISTR they have longer than standard spacing, so might not fit a
stand-off, without one I find the gutters are in my face.

Also, I understand that you extend them from the top, upwards. If I want
to use it at half height, will the bottom portion prevent me from using it?

I think you can extend the 'spare' rungs to the top or leave them at the
bottom, but then you have to step over them.