Hi there,
Ive got a Worcester Greenstar 24i junior, which needs to be wired in to a double pole isolating switch. My closest power point is a single plug point under the kitchen sink. Will I be able to tap into that point and run a new point up to the boiler?

Not certain where you are or enough details to give a good answer, but don't do it from the outlet. You should have a dedicated circuit from the breaker panel serving the furnace only. It can be done other ways but a dedicated circuit is required many places and is recommended everywhere else.