1.We are thinking of fitting the new combi boiler in the bathroom,do i
simply connect a mains cold water supply to the boiler and tap in to
our existing hot water pipes with the DHW connection.

Basically yes, provided you've removed and sealed off the old gravity
gubbins.

2.What size plastic pipes do you run for the central heating part,do
these reduce down at any point.

It depends on your layout. There are various choices. You can run to
manifolds and then take microbore from there to individual rads. You can run
a 22mm trunk circuit and take 15mm or microbore to the rad valves. You can
subzone into several zones and run 15mm for each zone. It all depends on
your house layout and intended controls.

3.Do plastic pipes need lagging or are they thermally efficient.

When possible, lag all pipes, whether plastic, metal, central heating, cold
or DHW. This increases energy efficiency, prevents freezing and maintains
hot water at the tap after a longer pause between uses.

4.I understand the need for a filling loop,what other stop valves or
relief valves are needed.

The combi boiler will normally come with all the valves required, or
describe which are missing.

5.Do all combi boilers have a pump internally or do some not.

The vast overwhelming majority have an internal pump. I can't think of a
counterexample, but I bet someone can search out some obscure example.

6.We are not planning on fitting radiators initially,can just the DHW
system be used then radiators connected afterwards.

Normally this is OK, although you may need to check if the boiler requires
some sort of heat dumping capacity in the heating circuit.

7.Will any of our existing water pipes(hot/cold)to taps throughout our
house need replacing.

If they do, then they were on their last legs anyway and you now have an
opportunity to search for failing fittings at your own convenience.

8.Do combi boilers come with installation/startup instructions for the
initial setup.

Normally, although this can require some specialist equipment that your
CORGI fitter will need to do.

9.After the gas fitter has connected the boiler,should he set the
boiler up and check the hot water system or is this left to yourself.

Let them do it. It won't take long and it means he can sign your benchmark
book and use expensive or unusual equipment for the checks (i.e. gas
analysers, manometers).

Christian.