Hopkins is right - It was a Marshall 'Micro Stack' with a 5150 and also a Anniversary edition Marshall JCM -900 head. How do you know Andy Sneap!?
They also used Marshall Guv'nor pedals (not the new ones, mind you!)
I would argue that all the other guitar 'effects' you hear on the Carcass songs would have been courtesy of Colin Richardson... Like the flanger/phaser type sound at the end of Symposium of Sickness etc.
I have an old Guitar World mag (from about '94) which had a main feature on Carcass/Obituary and there was an in depth interview with Bill and Mike Hickey... they both said that Carcass were not really in to collecting guitar effects pedals etc. and wanted to keep it simple. It was a great article if you can find it anywhere.