Going back to the original questions:
pault wrote:having read the previous postings about plumbing, am I right in thinking the following ...
a plumber should charge me approx £100 including drilling the work tops?
a half-decent plumber won't have any problems doing the job?
The plumber will have to tee into the rising main (usually found under the kitchen sink), install a service valve and check valve and simply route the new pipe to the machine. If you have a timber based worktop, he will be able to drill through it in a matter of seconds. If you have something more exotic e.g. granite or cast concrete then you may prefer to chop a channel in the back wall. 100 would probably be reasonable including materials but I think would be a bit high for labour only - depends on your layout.
is the "in line" filter already fitted in the machine - or does that need to be done by the plumber too, or is one not really necessary?
See Jim Schulman's water FAQ. Although tap water is supposed to be safe to drink, I have fitted a ceramic filter (from avonsoft.com) to my drinking water tap which I change annually and it's surprising how much it traps.
(forgive the basic nature of the questions - but I have been ripped off by a builder and want to avoid the same happening again)
Never admit total ignorance - make like a geezer. Ask if the price you've been quoted is a firm price or an estimate. Don't pay until the job is completed to your satisfaction although paying a small amount for materials at the start would not be unreasonable.
If you need any more help, PM me and I'll give you my phone no.
Ian