Thanks for that Chris, to be honest I thought there would be more products on the market for this and we'd find a host of things to try within minutes of my forst post, but it seems I was wrong.
If we get the job we could always refer that part to the lead expert that will be doing the hidden gutters and other lead work.
Thanks muchly for all the advice given here, again 'if' we get the job, photos and measurements will follow.