If you can find a produce dealer with a ripe 'San Marzano' tomatoe - chances are the seeds would be viable - though not necessarily true to type as you won't know if the plants were grown in isolation from other tomato varieties. (tomatoes are pretty much self-pollinated but not comletly - they should be raised seperately if saving seed to keep varieties true to type)

There are sources for 'San Marzano' open pollinated seeds in both the US and Canada - though I don't find any listed for the UK - if they say 'F1' or 'HYBRID' anywhere in the description they are NOT the variety you want.