As you're not flying during school holidays (although there is a half-term week in October come to think of it) I don't think you need rush to book as they won't sell out any time soon.
I've found from previous trips that prices on the internet tend to start to fall about 6 months before your departure date, so if you can wait, hold off until then. Although get yourself a price quote now so you can see whether they have come down or not (a mistake I made once - forgot the original price!)
As for multi-stop holidays, the big 3 websites in my opinion (Expedia, Opodo and E-bookers) all will do them, but E-bookers will only accept them via e-mail and then mail you back with a price. With Opodo it is all done on-line, but you have to choose what flights you want BEFORE it will give you a price. Expedia is the easiest as it will give you quotes on various airlines for the closest matches to your schedule FIRST and then give you the chance to edit your flight times to see if you can come up with something better.