We have a Honeywell Smartfit controller which keeps
crashing showing code F4 on the timer. F4 is 3-port valve
stuck but the valve and actuator seem all right. Google
suggests this is a common problem and the solution is to
replace the controller.

First question is, has anyone managed to get a free
replacement out of Honeywell?

Failing that, can I replace it by a different make?
Can I just connect up the boiler, valve and timer the same
way or would I need a new timer?

Is it even possible to DIY a controller, e.g. given a
microcontroller board and some relays? Being able to
fine-tune the heating program would be great.