Download the Network Setup app from the Marketplace. It should reset your MMS settings for you.
Is it available again within Nokia collection or pre-installed in Tango updates?
I was presented with an update for Network Setup after updating to Tango so I would have thought it is available in Marketplace as well.
What happens if you go to Settings > about > reset your phone - to factory defaults so takes settings from your PAYG SIM?
Good to hear you got it sorted, even if you had to resort to unethical means.
Just to let you know that none of my settings were affected when updated my phone. The update wnet smoothly and I didn't need to do anything afterwards. Perhaps something went wrong during your update. Main thing is you got it sorted. If yo don't mind me asking, what are you main and spare phones then?