Bulbs, cushions, blankets and rags aren't such good ideas for a winter
shelter.  Cats prefer dark cubby holes to cuddle up in; cloth retains
moisture and actually draws body heat out of the cat.  The ideal inside
material is straw or hay over a mylar space blanket.  Cats like to burrow
into the hay and cuddle.  The space blanket reflects 80% of the cat's body
heat back to the cat and also warms the straw or hay and keeps it dry.  This
is what I use in my winter shelters for feral cats.  The primary objective
in constructing a winter shelter is keeping it *dry*.  Never put water in
the shelter.

Since the OP is in the UK,  he should contact Harrison Insulating Systems in
Lancashire. Harrison makes an excellent child size emergency space blanket,
and better still, a double-coated Life Reusable Blanket.  This space blanket
uses a soft insulating material inside sandwiched between an outer layer of
Kelvatherm on both sides.  The outer layer reflects cold away from the cat
and the inner layer reflects almost all of the cat's body heat back to the
cat.   http://www.hisystems.co.uk/products.htm

Most commercial small dog and cat houses are too big or too high which are
too large to retain enough of the cat's body heat.  The shelter should be no
higher than 12".  Neighborhood Cats has downloadable plans for building an
excellent winter shelter.
http://www.neighborhoodcats.org/downloads/shelter_plans.pdf

The only additions I made to the shelter is covering the outer side of
insulation material with plywood and covering the roof with shingles.  I
tried using cat flaps, but some cats won't use them.  You could try tacking
thick pieces of clear plastic over the opening- one strip every few days-
*after* the cat starts using the shelter.

Phil