Combi timer

Any timer will let you control a relay to give volt free switching. All you need is a solenoid relay that works with a mains voltage coil on the solenoid. They're not to big so could probably fit inside the boiler near the original timer unit. It all depends on what Ampage the contacts need to be rated at for the boiler control.

You're looking for something like these on the top of row of pictures:

Just get a heating timer with volts free contacts. Even a bog-standard timer like a Honeywell ST699 has them, so I expect more modern ones do too.

Or go for a programmable thermostat instead, leave the boiler timer set to permenently on and then you can tell the stat what temperatures you want and when (rather than the crude "on or off" control you get with a normal timer)

Set the boiler to run continously, and wire in a Honeywell CM67NG RF controller, put the reciver close to the boiler, the main control unit in an approprate place in the house.

That looks like the hardwired version use it to replace your current room stat, frig the timer so heating is on constant and use the cm67 to control the heating.

They mean use a programmable thermostat and don't bother using the onboard timer. Alternatively, use a conventional thermostat and a separate timer, but this has loads of disadvantages and even costs more.

A search for "programmable thermostat + uk" will reveal about 500 models to choose from, from as many suppliers.

Christian.