Combi boiler thermostat not working properly

I have a Worcester 28i combi boiler with an attached Sunvic TLX thermostat and things are working ok... but then not really.
The boiler is working fine. No problem.
The thermostat should turn heating on and off to keep room temperature within a tight range. Mine doesn't. At the current setting the boiler doesn't start heating until the temperature reached about 17 degrees and it doesn't switch the boiler off until room temperature has reached about 27 degrees. The result is that I manually use the dial on the thermostat to turn heating on and off. Less than ideal!
What should I do?
I hesitate to replace with another Sunvic TLX (or equivalent) as I fear the same results. Somehow believe a digital thermostat would do a better job but I don't know which are compatible with the Worcester 28i.
Thanks for any comments.
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Can I just add...
The ideal solution would be a radio controlled thermostat as the current position is not ideal. But this would require the Worcester 28i to be compatible I suppose.... ? Is it possible?

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bobby wrote:

Excellent but not cheap, should work with any boiler.
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I have the same boiler and recently fitted a Sunvic TLX stat also. I like yourself found that the swing in temperature between on/off was too great. I took it back and replaced it with a Drayton Digistat which is much better the temperature now remains at a constant 22 degrees which I have it set at after initial warm up and it does not overshoot during initial the warm up also.
Marc
bobby wrote:

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I'd always use a digital thermostat, never an old mechanical one. They are compatible with just about any system imaginable capable of using a switched thermostat (as opposed to proportional/analogue system).
I'd recommend using a programmable thermostat. You can then set your combi programmer to always on. This is particularly useful if the main programmer is inaccessible, or installed in an unlighted cupboard, so needs a torch. They are also useful if you have zoned heating.
Christian.
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Thanks for your comments Bill, Marc and Christian. Very useful.

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