My existing floor standing boiler has now died on me and I have
now the opportunity to buy and install a more efficient boiler.
The problem is that the house (1901 circa) has the original single
pipe system (mild steel pipe work) that is gravity fed and runs
throughout the whole house servicing 20 rads.

I'm afraid that you may need to bite the bullet and replace your pipework.

This will, though, have the advantage of enabling you to subzone your
heating requirements. You should, at the least, have zones for the bedrooms
and living areas (and preferably the kitchen). This way, you don't need to
heat your bedrooms during the day.

Basically, much better to throw your money at bringing the pipework up to
date than waste it on inefficient old-fashioned expensive commercial
boilers.

Make sure you do it in S-Plan plus, with sufficient sub zones, as stated
above.

Christian.