You'll more than likely be on 15mm copper, if you have an isolator on you won't even have to turn the water off, just T into using plastic/compression fittings and swap the waste pipe on the sink for one with two inlets if it doesn't have them already, depending on how the washer waste is done.

Dishwasher just needs a cold feed and obviously somewhere for the waste water to go.

That all sounds fairly straightforward to do whichever option I go with, I was worried I'd have to get a plumber out to do some welding or some shit like that! :lol:

You can buy a plastic 'Y' piece from B&Q etc

Very easy, specially if all it needs is a cold feed.

Are the pipes copper? If so then just cut into the pipe and put a compression or even push fit T with an outlet for a dish washer

If there is only plumbing for one appliance (washing machine), how do you go about modifying it so that a dishwasher can be used as well?

Is this something you can do yourself (i.e. is it easy DIY) or is it best left to a plumber?