Can i easily keep the current indirect heating system used for our hot
water, while replacing the central heating section. Can a
combi/condensing boiler do indirect water heating efficiently.

Yes. No problem whatsoever. Go condensing whatever you decide.

If you choose a combi boiler in a kitchen location, then you can use the
mains DHW outlet from this for the kitchen sink, whilst still heating the
cylinder. This could be useful if the kitchen is far from the cylinder
location. However, there's no real need for a combi if the indirect system
gets enough hot water to the right places in the right amount of time.

If someone could explain the need for room thermostats when you have
trvs i would appreciate it

It is quite simple. The building regulations require you to have something
called a "boiler interlock". What they mean is that the boiler must totally
switch off when no rooms require heat. If you had TRVs on every radiator,
the boiler will happily continue to pump hot water through the bypass,
merrily wasting gas.

The room thermostat solves this problem by having a master room with a
TRVless radiator. When this room warms up (and it is set to warm up ever so
slightly more slowly than other rooms), it turns off the boiler. When it has
cooled a little, the boiler fires up again and other rooms can top off with
heat too.

Another solution is to have TRVs on every radiator, but use a flow switch to
detect that the TRVs are actually passing water. When the flow switch
indicates that all TRVs are closed, it turns off the boiler. The pump is
left running, or is pulsed every 10 minutes are so to see if the TRVs ever
open again.

Types of boiler/boiler recommendations that fit in with our budget
would also be appreciated.

I'd go for a inexpensive, but not too cheap, modern condensing system boiler
such as an Icos or Glowworm, plumbed in S-Plan.

Christian.