When I had my M3 delivered the guy told me not to use an extension lead on the 3 pin plug
Yeah, that's the Party Line. Here's my 2p-worth:
Get a good quality extension, like the one
@VanillaAir_UK links to, not an El Cheapo from Screwfix / Homebase, nor your mate's frayed lawn mower extension!!. Something with have H07RN-F rubber cable rather than some nasty PVC stuff, so that it can withstand a fair amount of abuse scraping on tarmac etc. and continues performs well in sub-zero temperatures (blue "Arctic" PVC extension cable is rubbish ...) and a rubberised plug / socket. I bought mine "one cable size up" so that combination of extension and UMC doesn't stress anything (but Tough Leads blurb on extension leads says that is not necessary, and it bulks up the cable weight, and increases the price)
You can waterproof the connection to UMC with bucket and two bricks - connection on top of a brick to keep it dry, upturned bucked with brick on top to stop it blowing away ... and better still buy something with a more robust solution for socket-end (they won't be waterproof though, only splash-proof, so you might still want Bucket+2-bricks
)
Feel the plug (at 13-AMP socket end) for any warmth after a hour or so, and before going to bed. Be prepared that M3 may trip the whole house breaker ...
If any warmth dial down the AMPs on the car dashboard, or try a different socket.
Decide if you need RCD on the plug end (circuit may well be modern / well protected ... but others in future might not?)
Don't precondition on 13AMP (until final few minutes). 13AMP isn't enough to do that and charge as well
Tesla UMC is a robust cable (capable of 3-Phase), other "granny leads" are much thinner. I've not had any problem using an extension (and by the nature of it connection time has been all-weekend continuous running)