Hi Gillywigs and welcome to GoY you would be far better to remove the mossy turf before you lay the weed membrane. You can turn the turves upside down in a stack and allow them to rot down for 5 years or so - you will then have wonderful loam. Oh and whilst the weed membrane will stop, for a short while, the weeds growing from below the weeds that sow into the gravel will still grow and need pulling out. Within a few years the membrane will start to deteriorate and the weeds will be back with a vengeance!

Thanks for your response.
I thought it would be a better idea to dig it up but was hoping for an easier option! I think in the long term you are right. I have to say GoY is great. I only found it last night whilst looking for pics of gravel gardens on the net. Thank you for welcoming me!