position of room thermostat

I bought an old house with an oil central heating system & exterior boiler which I have been advised is too big for the space it feeds - the new room thermostat is positioned on boxing in over pipes (H &C)
The ambient temperature is COLD being close to the water table.
The radiators have separate temperature controls.
So the problem is.....
It takes no time to reach (thermo) room temperature then cuts off for ages &
I often have to keep adjusting up the thermo.
My fuel bills are outrageous & I'm dreading the winter.....
Any answer to what combination off these factors could help resolve the problem?
Many thanks

housemartinette said:

I bought an old house with an oil central heating system & exterior boiler which I have been advised is too big for the space it feeds

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Some oil boilers can be range rated to give a lower output. Which boiler do you have (make and exact model no)?

Do you know what size boiler you actually need? If not, use the Boiler Size Calculator to find out. This only gives the heating requirement, if you have a combi boiler it will be much larger.

the new room thermostat is positioned on boxing in over pipes (H &C)

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So the stat is reacting to the temperature of the water in the pipes??

The ambient temperature is COLD being close to the water table.

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That's a new one to me! How far below ground is the water table?

The radiators have separate temperature controls.

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That's one good thing

It takes no time to reach (thermo) room temperature then cuts off for ages & I often have to keep adjusting up the thermo.

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That's due to the location of the room stat.

Thanks for feedback D-Hailsham...

some more info:
1. The boiler is a Grant Vortex Outdoor Combi 90V.3
2. I was hoping it was near-pipe position - simplest to sort
3. About 6 inches - the ground floor has a new dampcourse/raised floor

The ground floor also has a fireplace & log-burner both working & used
(because of expense of central heating oil)

could I put some heat reflective material between pipes/boxing in/thermo?

Thank again