I always assumed you could drive an MOT failure home or to a garage for repair in England Pat? Maybe this is something new.
We have the NCT (National Car Test) for years now here, much the same as the MOT, but you have to bring the jam jar to Official test centres, while you can get your MOT almost anywhere in the UK and at short notice, we have to book for weeks in advance.
I'm assuming you're using Halfords or Holts rattle cans or similar? You'll need to get all the wax off it before you clear it. Now, I never had to do it but hot water and Fairy liquid and an old toothbrush should shift it I'd imagine. I'd spoon test everything you've already done, try to get it off and see how you get on. Maybe Geoff knows, if he's looking in.