How recently was the work done? My first thought is to get the contractors back as they almost certainly cut corners by not properly clearing the ground.

It was done in December!

I have had a membrane down in my garden for 8 years and have not had weeds come through it, so go back to the contractor and complain as was said above they may have cut corners.

Thank you both Urbanite and John743 for your replies. I'm very new to gardening and wasn't sure if this was normal or not, but considering how much I paid I didn't think it is. I will contact the company and question their work.

Again thank you - Lorraine

Some very strong weeds like horsetail will penetrate membrane but most won't. If you can add a photo we might be able to identify what they are - do you have very neglected gardens on either side of you?

Almost certain that the contractor didn't properly clear the ground beforehand. Before you complain, though, you need to check on what they quoted for (and what you asked for). If you didn't ask and they didn't quote then you probably don't have any recourse. But if you asked and they quoted for clearing the ground then you should reasonably expect not to be picking weeds within six months of the work. Also, what depth of gravel did you specify? It needs to be at least 3-4 inches deep.