Somehow believe a digital thermostat would do a better job but
I don't know which are compatible with the Worcester 28i.

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.