I find the easiest approach to my work standards is by asking myself if I would be happy with it in my home??
I would never be happy with a socket run from anything other that it's own ring main, I want my cooker on it's own, my boiler on it's own and my lighting on it's own as a very minimum. I have the kitchen on it's own circuit and outside lights on their own.
In theory, using the feed to the boiler would work fine. However when most electricians are installing a circuit and they know the load demanded on it, the materials used tend to be for that specification. To then go and make changes on a circuit which has not been installed with this in mind is (in my opinion) a bad move.
What would you think if when looking to buy a new home, you saw sockets fed from a boiler feed???
Taking a look at home this evening, I could confidently take a spur from the back of an existing socket in an upstairs bedroom, channel it up the wall, fill it and add a socket in the loft with no more than 3 hours work. O.K the cost's are higher but so is the standard of work and isn't this what shows the pro's from the DIYers?
We have a saying in our house..... "cheap work, cooks people"...........we'll not really, but I might just start it off!!!