The biggest you can fit in your frame and on your rim, would recommend 35 or 38 but most frames like ours on paper only have clearances for 32 at most. Marathon Plus come up bigger than most tyres also. Give University Cycles a try, he normally has a stock of them quite cheap but sometimes odd sizes. You can probably get away with a regular Marathon on the front and they are way cheaper.
The biggest you can fit in your frame and on your rim, would recommend 35 or 38 but most frames like ours on paper only have clearances for 32 at most. Marathon Plus come up bigger than most tyres also. Give University Cycles a try, he normally has a stock of them quite cheap but sometimes odd sizes. You can probably get away with a regular Marathon on the front and they are way cheaper.