I suspect some clot has overtightened it and/or some clot has tried to undo it with a sub standard 16 sided socket (or even the wrong size), instead of what any self respecting mechanic would use on such bolts, a 6 sided socket which won't slip and chew the head up.

Service manager first call, second call dealer principal, still no joy inform Toyota UK of the poor customer service of their service dept and ask them to get involved.

I had the problem of a stripped thread on one on my 406s, the solution I found was to wrap some PTFE tape round it before replacing, did that every subsequent oil change and had no problems for 50k+ miles.

A new sump plug and washer is under a fiver. If you for one moment consider the advice above

you should insist they fix it at their expense. If they refuse, I would consider towing the car back home for an independent inspection (e.g.AA), and having it fixed by someone else and suing them via the small claims court for the cost (and the cost of a hire car) if they refuse to pay.

think again. The cost would be many times greater than simply buying a new plug and either fitting it yourself or letting them fit it.

Then find yourself another garage.