It's mostly about how much hot water you want at once, not how many radiators you have. A combi's size is its hot water output, and your water supply sets the limit on what any combi can give you.
If your heating doesn't have one, almost certainly yes. A magnetic filter catches the black sludge that wears boilers out, it's cleaned at every service, and Worcester Bosch's longer guarantees depend on one being fitted with the boiler.
Test the heating before the cold arrives, check the pressure, bleed the radiators, protect the condensate pipe and get it serviced. Ten minutes now can save a cold week in January.
Open doors and windows, turn the gas off at the meter, don't touch electrical switches or use naked flames, and call the National Gas Emergency Service on 0800 111 999. It's free, 24 hours a day.
Turn the boiler off, contain the water and keep it away from the electrics. Then work out where it's coming from: a drip from a pipe outside is very different from water inside the casing.