LEAK UNDER FLOOR. BOILER ON ALL THE TIME AND WHAT IS PART IN PHOTOS?

[Dimmy007]

Some advice needed please re a 3 year old combi gas boiler. The hot water works but no heating. A gas engineer was already booked to do the annual safety check before the fault developed. He said the boiler is ok with no fault. He said there must be a leak under the floor boards causing hot water to escape and that is why the boiler is contiually running as it thinks a hot tap is open. I have 2 questions please..
1. If there is an underfloor leak shouldn't the boiler pressure guage be dropping? but it is not..it is constant at 1.5 to 2 bar.
2. The engineer left this part behind after he did the safety check (see photo). Is it part of the boiler?..i have pictured it from both sides..many thanks!

 

[Sparkgap]

Can't tell what the bit is, but the underfloor leak is probably on your hot water side, not heating. Don't know why the heating is not working but the pressure gauge measures the heating system pressure. Simple way to find out is shut off the hot water outlet valve from the boiler (NOT the heating valve), if it stops then Robert is your mother's brother!

 

[snowhead]

The part left behind looks to be a cap for the boiler flue test point.
Is the Boiler a Worcester?

Should look like this when fitted like this on the top of the boiler,

 

[Ricky]

Fits back on flue on top of boiler