T i m wrote:

Ok .. understood. Do folk still install 'traditional' 'tanks in the
loft and cyl in the airing cupboard' type setups?

Yup, or even both (and even the boiler) in the loft.


ie, if we were to
stay with the main system (it's all 1-1/4", 3/4" , 1/2" .. it has all
been there a while but (currently) has no problems) could we simply
fit a 'modern' high-efficiency boiler?

Yes