1/ Anyone have any experience of these combi boilers? Are they any good?

They are well regarded.

2/ How do I determine my BTU to determine what size boiler to select?
(3 bed semi, 7 radiators to feed).

Except for very large leaky houses, even the smallest combi boiler will
suffice for almost any domestic property not owned by a footballer or chief
executive. The size of the boiler is high in order to provide sufficient
instantaneous hot water. This requirement is usually very much in excess of
the heating requirement for the house. A typical 3 bed semi would require
about 5-15kW depending on the insulation levels. A combi is typically around
the 28kW mark.

A 3 bed semi even with no wall insulation and a small layer of manky dodgy
loft insulation would not trouble any combi boiler. Generally speaking, if
calculations show that a larger boiler is required, it should be reread as
stating that the insulation needs improving instead.

Christian.