copper pipe replacement with new boiler

Hi,
I've just had a Potterton Puma 80 boiler replaced with a Worc Bosch 24i (literally today).
When I shopped around for a few quotes, some gas fitters mentioned that I would need to get the 15mm pipework replaced with 22m.
Apparently, the gas pipe from the outside meter is 22mm as it goes into the house but the pipework coming down from the ceiling into the Potterton looked like 15mm.
The gas fitter I went with didn't mention this 22mm requirement and said he would join up to whatever pipework was there.
The house was built in 1996 so not sure if it already had 22 mm under the floorboards but am now a little concerned after listening to a few people that this boiler installation is not legal?
The house is very small - 1 bedroom, 4 radiators hence the reason for a 24i boiler.
I suppose my question is - what's the worst thing that can happen with 15mm pipework left in.
Thanks.

If the boiler needs 22mm and it's only got 15mm it may underperform, and may suffer other issues. A good RGI will calculate the pipe size required, this varies on every job as it depends on the length of the run, how many elbows or bends there are, and whether there are any other appliances connected to the system.

As a general rule a combi will need 22mm, having said that we're installing one at the moment that's so far from the meter that it needs 28mm for part of the run, 15mm would generally be undersized for a combi. There should be a max 1mbar pressure drop between the meter and the boiler, if it's more than that then the pipe is too small. This check is part of the commissioning process and as such the boiler can't be commissioned correctly if the pressure drop is greater.

Installers that choose a WB 24i often cut corners. The answer whether or not the 15 mm is sufficient can not be given online, it needs a test. I strongly doubt it is enough though.