condensing boiler

In the summer I replaced a massive Warmflow Bluebird 200 oil boiler holding 50L with a Grant Vortex 36-46 holding 21L. As its colder I have now switched UFH on but boiler short cycles every minute once up to heat. There was no short cycling with the Warmflow boiler. The UFH is set to 45c flow...

Discovered, on getting an engineer in to fix what I'd thought was probably a minor fault preventing the boiler from working, that it has had to be condemned (and yes, justifiably - you should see the insides! Ye gods ...). The house (probably 1000-odd sq. ft.) is a fairly typical 3-bed layout...