I'm sure everyone here knows just how good these two are - but just in case you don't:

Sylvia is an excellent Duet player, and Keith is probably unique in that he's brilliant, and equally virtuous, on both Anglo and English (how's the Duet coming on, Keith?). Both are very strong singers too (Keith is one of the very best in the business - a true master) and their cohesion is impeccable. Sparkling entertainers too.

If it's too late for a guest spot, have them on a singaround night and ask for a whip round. Then put 'em on first. No-one will want to follow them, and they'll all be chucking tenners in the pot with abandon. It'll turn onto their gig, but no-one will mind!

Tom