If I make/decorate a sponge cake today (Monday) (gonna try the staggered look,hopefully!) will it still be fresh and OK for eating on Friday?

Are you talking about a angel food type of cake or a pound cake type of cake? A pound cake might be okay because they are pretty moist to begin with. I think that making and decorating today for a Friday delivery might be pushing the envelope a little. The icing might be really crusty and starting to dry out. Can you bake and crumb coat today and freeze until Wednesday night and then take out and let defrost in the refrigerator and then decorate on Friday?

Was thinking of a Madeira cake

I have made a cake on a sat and covered in fondant the next day, finished off etc in the week and it was eaten on the following sat and I was told it was fine. It was made for a friend but I was having a back op on the Weds so the latest I could offer was the saturday before and she said it tasted as good as my freshly baked ones.
Also Lindy Smith states in her book a week to ice/decorate the cake and then a week to eat so if she says you can I guess you can

Thankyou, I would have thought it would be ok, just wanted 2nd, 3rd, 4th opinion!!

If you can then make the cake today, freeze it and then decorate the day before delivering. It breaks up the work load and seems to make it more manageable while keeping the cake fresh.