Went to the kitchen shop yesterday to buy a tin 12x9" but met the most brilliant assistant who was really interested in what I was doing and had trained as a pastry chef Just what I needed!
She advised me to use my grill pan (only used for cheese on toast so nice and clean) so that's what I've done, using Queen Mary's recipe for standard traybakes. She also agreed with making it today, decorating tomorrow, and Saturday for if it takes longer/things go wrong.
It's cooked and looks fine, and it's been brushed all over with sugar syrup, as advised, to keep it nice and moist.
I've made it vanilla; once cut into shape it has to have a 'crumb coat' of buttercream under the fondant, so I bought a jar of lemon curd thinking I'd flavour it with that, but what does anyone think? Will that be ok with a vanilla-flavoured cake?