Good news, the guy came out and had a look for me. Said it is not bad enough to warrant a entire new arch. He will cut out the rust and fabricate a piece to put in the bottom and same again for top of arch. £200-300 was what he said.
he had a good look underneath, had the side skirt off and looked all round the car and said he was amazed at how tidy it was which was good.
so I'm quite pleased with that