Looks like I definitely got a dud frontpipe.
Further anaylisis today showed the front nut for the first sensor to not only be out of position but also an incorrect thread to match the sensor. Due to it not fitting I've decided to renew both sensors at a cost of £120 seeing as they are both rusted and one almost threaded. This is a safer option as new sensors should work better with the new exhaust hopefully throwing up no Ecu faults.
On a positive note a second check showed the rear lambabda sensor should fit despite it being slightly tight.
So far this job has entailed , cutting and welding on a sports cat , red driling turbo bolt holes , relocation of front lambada sensor , purchasing new sensors/nuts and extension of the centre section.
Sometimes It's just never straight forward lol , really hope this is worth the hassle
