"I don't paticulary want to mix my own adhesive, so if possible I would like ready mixed. The reason is that I can start and stop tiling whenever I like without having to clean/mix more adhesive."
You can't use tubbed addy with natural stone tiles and expect them to stay put. You must used bagged cementatious flexible. If you are concerned about pot life use Mapei Keraflex. Not cheap but it'll last about 8 hours in the pot rather than 40 minutes. Open time is good too.
"I have been to B&Q and homebase to find a suitable pot of adhesive but to no avail."
You won't. Go to a proper tiling supplies outlet and ask them.
"One kid in B&Q suggested I use "multi purpose wall adhesive"
ROFLMAO - where do they find them? Have you got that in writing for the claim when they fall off?
"but I am dubious as to if it would be suitable."
You are correct to be so - 'cos it ain't!
"So, my rather simple question is, whats the best type of adhesive to use for this?"
If you want the tiles to stay on the wall you must use what you don't want to I'm afraid - or pay someone to do it for you.
"Indeed. I went to a local tiling outlet, and they said, within reason, I can use normal tile adhesive."
The key phrase being "within reason". What thats really saying is don't come back in here with a bleat if it don't work because we'll take no responsibility!
"Slate is a little different to other stone, as it is quite porous, and therefore the adhesive can "grab" it fine."
My answer to this would be if you were paying me to do the job and told me you wanted me to use tubbed addy from Bargain and Quick I would decline the job. You can take risks when in your own place and you might get away with it. When its a remunerated task and they fall off you get called back its at your expense not the clients. My advice doesn't change. You are free to follow it or not - your choice but at least if you come back here and say they fell off I can't be blamed for telling to use tubbed carp. If they stay up thats OK too.
"For example, if we were tiling with Marble, then it would be a different story."