Also there are no room thermostats although I have fitted thermostatic
valves to every radiator, so do I actually need one ?

You need what is termed a boiler interlock. This means that once no heating
is required, the boiler must be turned off. This can be done in two ways.
Firstly, you can have a room thermostat in one room with no TRVs. Secondly,
you can have an automatic bypass valve with a flow switch. When the
radiators are all off, the automatic bypass opens, the switch trips and the
boiler turns off. Some logic must be incorporated into the system to notice
that the valves have opened again, though.

If possible I would like to do most of the work myself and get a CORGI
man in to do the final connections into the boiler. Any ideas how
realistic this is ?

You can. Finding the CORGI can be problematic.

Can I re-use the existing pipework or will I have to run new pipes
throughout the house ? Do I still need a separate pump or will this
be built into the new boiler ?

Modern condensing and combi boilers frequently have internal pumps. It is by
no means universal, though.

Should I consider doing this in two phases, i.e Central Heating first
and Domestic Hot Water second ?

You could do this. You could also use the combi section for the kitchen and
shower and maintain stored water for the bath for higher flow rates.

Christian.