I've a solution....it was sitting down the side of the sofa all the time I was gazing at these expensive Brompton bags. I found an old lap top case which I'd bought for £2 from a charity shop. More or less dead on the correct size for the Brompton bag frame. Bag tied on and it looks like it was made for the job, even has pouches at the rear that appear ideal for water bottles! Various zipped compartments, bit flap over the top to weather proof(ish) it, shoulder strap, etc. Will take it for proper test ride this week and post some photos if it all works OK.