Only buy a genuine sensor from a genuine VX dealer that you genuinely have to walk in to.
I had exactly your symptoms recently with my 2.6, so I fitted a brand new genuine VX crank sensor 90540743 that matched the Bosch no of the faulty one I removed (the replacement I had previously bought from Vauxhall), but after fitting it, it fired up and ran for a short period, but then it refused to start again ( just cranked). Ended up fitting a known used genuine Bosch sensor off one of my 3.2 breakers (which was obviously a previous replacement as I could get it off in one piece) and it has been fine since.
Although I agree totally with the sentiment about getting a genuine VX from a VX dealership, this isn't 100% foolproof as I found out! The same genuine Bosch marketed product is part no 0 261 210 131 if that helps.
