I'm a big No.5 fan too but can't afford it either.
Before Christmas I bought a perfume in Lidl that is really similar. Not in the UK though and it doesn't seem to have been sold in UK. It's called Suddenly Woman 1 - not the Madame Glamour one that is normally in stock.
I haven't tried any M&S perfumes but in Next I like the Cashmere one. I've had lots of compliments on it.