In anticipation that it will rain one day, we are putting in another water reservoir on the last remaining downpipe that doesn’t have one.
For various reasons we want to use the connecting kit that involves drilling a hole in the down pipe, rather than cutting out a section. We’ve done this twice before using a scie cloche (don’t know english name as never used one in UK) and can’t remember how we stopped the circle of pipe from falling into the downpipe and blocking it.
Any ideas!?
If you can use the hole saw at a slight angle so there is a small portion of the circle still attached then flex the cut disc until it comes away.
Glue a length of thin string to the centre of the cutout. I’ve done this before. It’s a bit awkward as the string can get in the way, but it works.
‘Hole cutter’
Have just discovered that the French also use trépan - though can’t believe they still practice DIY trepanning.
Alternatively clamp a mole wrench on the nearly cut-out circle to stop it falling into the pipe and finish off with a key-hole saw or similar.
Ah. Hole cutter. The string trick doesn’t work with hole cutters. If it’s the drill mounted type, then the central drill bit along with the circular cutter will hold the plastic plug you’re cutting out.
Not plastic down pipe, but zinc…
Pedant here. I’d use the term “hole saw” in English.
Random example…
scie cloche= hole saw.
Lidl and Aldi are both likely to offer kits containing a set of different sizes in a box in next couple of months or so, they all seem to have been good quality Made in Germany ones in the recent couple of years.
English ‘English’?
Hole saws – also referred to as hole cutters or hole drill bits
Not by engineers 
more by people who dont work around that sort of thing.
Ha ha… just asked OH… “trépan” is what he came back with… he has a kit with various sized “bits” for when he needs to make holes of different sizes… and for wood or metal…
but he reckons hole-saws will find what he wants in English…
Hole saws, hole cutters - both horrid names. I’m going to stick to the French which is so much nicer.
It would matter not if it was plastic, zinc or mashed potato, using a hole saw the plug will almost, 99.9%, of the time the plug will remain within the body of the saw. In more than fifty years I have never lost the centre of the hole.
There is (usually) always a drill bit in the centre to guide the “trepan” and the bit you cut out will stay sitting on the drill bit.
Except on the rare occasion you dont want it to. 
But yes mostly do.
If it falls in so what, it might increase he new outflow
The more usual difficulty is actually removing the cut circle of material from the scie cloche.