Thanks to all for the responses.
It looks like the answer is a Worcester Greenstar running vented into a
direct heatbank with a zone valve for the rads and a second zone valve for
the towel rails and airing cupboard rad. The boiler and cylinder (of
whichever type) will be in the garage so the F&E tank will need to be in the
loft and the DHW will need a return pipe to ensure hot water doesn't take
too long to reach the taps.

Any other comments?
Dave S