1994 Symphony oil drain plug

Went to change the oil on my 1994 Symphony but couldn’t figure out the size key I needed to undo the drain plug. It looks like it should be an 8mm square key but after buying one from Halfords it does not fit.  It appears a fraction too large or almost as though it is not quite square?

I've come across this before - I can't recall what vehicle it was but it seemed as though the square "socket" in the plug was tapered. My (rather crude, perhaps) solution was to take an angle grinder to the end of the bit and slim it down so that it fitted. Ok, it takes the chrome off but that's no big deal.

It may be worth buying a new, correct replacement sump plug in advance in case you have to mangle the old one getting it out!

Thanks mojo. I will give that a try and have already ordered a new plug in advance.

We change the sump plug washer every oil change, sometimes, if hard to remove, the sump plug itself as well.