Bobplum - a flange is a big octagonal (?) nut thing wot goes on the back of the thread. They are quite heavy duty often out of stock at toolstation and IMO not needed.
You can also get a smooth round lighter one which also does the job fine.
You can also cut the hole a fraction small and wind the immersion in cutting its own thread with no locking ring at all.
The middle option is fine as long as you can grip the inner ring until it starts biting into the plastic. (me holding it with rubber faced builders gloves while my son in law tightened the immersion worked fine for me)
But!!!!!! Do not use an adjustable hole saw - get the right size or you may find the arms bend and the hole is not cut truly round and you get leaks which you can't seal and you spend two days faffing round with heat guns and glue and sealants and end up having to buy another tank anyway!
Do not ask me how I know this!
On a lighter note I have cut three holes in three new tanks and fitted three immersions successfully and all are still fine leak-wise.