only smarties have the answer .
from looking at the web site i would say valiant boilers, from experience push fit plastic pipe, most rads have a 10 year guarantee.
i would get them in for a price out of interest. they will offer you a deal if you say no. they also want you to sign up the same day and will offer you finance saying that the savings over the next 5 years will actually pay the finance off.
one offer they were doing years ago was if you keep all your gas bills and reciepts go back to them in 10 years time and they will give you 100% cash back .. usually backed by a leading bank.
as i say it cant hurt to get them in. they usually letter an area say 10,000 homes and expect 3 % interest so the estimator will be in your area ..