Got around to de-cat and centre box removal today. On close examination I decided to punch out the cat ( thanks darren mk )as it was impractical to cut it out of the system. Replaced the centre box with sleeved 60mm steel tubing. I have pics of the whole operation and can do a step by step if it is of use to anyone. The whole exercise comes in at less than £20. I cant run to a full stainless at the mo!