trike-type-seat that attaches on back of a pram/buggy?

[Jacksmybaby]

I have seen these around and want to look into getting one for my 3yo for when new baby arrives (due Oct)... but no idea what they are even called let alone where to buy one...

Does anyone know the type of thing I mean and if so:
What are they called?
Where to buy one and how much?
What types of buggy/pram will they fit on?
Would it be suitable for a (HEAVY) nearly 4yo?

TIA

is it a strolli that you mean? I have never used one but have read some good reviews

Brilliant! Thanks so much asuwere.

Has anyone else used one and if so what did you think?

it wasnt great on the mountain buggy, it kept flipping under.
the kid-sit is much better if you have a back bar on your pram