Tony,
Welcome to the learning curve

Don't dwell on the negative, hold on to the positive - you have learned something. 90% of indian restaurants are a waste of time unless you are a cheapo splash n dash outfit.
However I think your sales approach is wrong. You need to AVOID PRICE.
Go in and ask them a direct question... "I'm a carpet cleaner and I do free demonstrations, when can I come in?"
Obviously the next question they will ask is how much do you charge. Avoid it, tell them you will only know after you've done a demo.
As soon as you've got the chance to make a lovely clean square patch on their greasy blacktopped carpet then they can see why you will get it a lot better than their £60 cheapo man or waiter with a rug doctor. Then you can talk price because you have the ultimate bargaining tool. Start high and do not drop lower than the price you want for it.
By the way I'd avoid the belgian wiltons like the plague, simply a non starter, let them ruin them on their own.
Realistically you might only get the agreement for a free demo from 1 in 10, maybe 2 in 10. But they could be customers for a long time if you get it right.