Ian
Instructing is great fun and very rewarding at the right schools and with the right students. It's true the money is terrible and most places will only pay you for the hours that you fly.
If it's a bad weather day and your students want to come and do some ground school then you'll find at most places you wont get paid for it.
The going rates I have found recently is from £10 - £20 per hour flying for teaching PPL, maybe a bit more for IMC if you have an IRI to teach it.
Remember that in the summer if you work your ass off you will make the money. In the winter with short days and more bad weather and even unIMCable weather than good, the hours will rapidly come down and so will the money.
I'm not saying don't do it. I love it and have alot of fun, but I also have a business to keep the proper money coming in.
One possible way to go after you have a bit of experience is into a Commercial Flying School. Much better money in that line of work!
Good Luck and enjoy!