We’ve been told our builder hasn’t put the flu in right. Where can I find out if he has or not to check it?

Look for your nearest HETAS installer who can inspect it. All boiler flues should be a HETAS approved installation I thought.

You could have a go at diagnosing some issues from the building regs if you like (NB regs arent the same as hetas guidelines):
http://www.planningportal.gov.uk/uploads/br/BR_PDF_ADJ_2002.pdf

what’s he not done “right” then and who told you?

another builder told us that the outlet/flu goes out onto a neighbours land and so either needs re-routing or extending up the building further.

see page 43 of the regs, table 3.4 and table 4.2

http://www.canterbury.gov.uk/assets/buildingcontrol/approved-document-j-2002a.pdf

(Planning portal is down for me so use this link)

The op dose not mention solid fuel Stoner, this could be a gas boiler issue?

If this is a solid fuel boiler Part J of the building regs I think. Hetas is only needed for self certification. The other builder may use building control.

Probably just needs a plume management kit. What model boiler is it?

baxi duo tech 28

building control restrictions still apply to gas flue locations at boundaries.

Gas Combi? Most installation guides are like this:

Flue Postioning

Something like this

http://www.directheatingsupplies.co.uk/boiler/baxiplumedisplacementkitincflue5118638/

600mm horizontally from boundary edge rings a bell. can be taken up the face of the building but there’s normally a maximum length of flue run