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.