I've changed the valve on my dinghy deck. Followed the Polymarine instructions to the letter and had no problems with the job. Bit nerve wracking cutting into an otherwise intact bit of kit but it was easy enough to do and looks and works fine.
Yesterday took the plunge and cut out the old valve, made very small incisions and forced new unit in. Have left glue to cure until the weekend with just enough air to retain tube shape.
Thank you for the advice.