Which Central Heating Programmer for combi boiler?

Hi All

I've just had a combi boiler fitted and wanted a separate central heating programmer fitted (as opposed to the one built-into the boiler).

I'm looking for one with a specific function. I've seen some of them have a 1 hour advance/timer button which when pressed puts the central heating on for exactly 1 hour. However 1 hour is too long so was wondering if there's such a programmer which let's you adjust this value to say 20 mins?

I've had a look around and can't find one, so thought I'd pick your brains to see if there is such a thing??

Thanks in advance.

no

OK, seems odd, maybe someone should invent such a thing, where this advance time is variable, as opposed to just hard set to 1 hour.