Cards in shop windows and notice boards works well.
If you're doing 11+ prep, then it's cards in the newsagents, post offices, spar shops, garages, etc around the primary schools where the parents will be shopping before/after the school run.
Most of the uni campuses we've visited have tutor cards on notice boards all over the campus and nearby shops.
Slightly harder with GCSE/A level as it's more likely to be parents wanting to organise it, and they'll be spread all over miles around the school. You could try cards in shops/takeaways close to the school but that would just aim at students themselves who may not be the motivated ones to organise it!
Other than that, a simple free one page website with meta/key words such as "maths tutor Southport" will get a few google hits. And of course, a facebook page, again with short, snappy key words.
Once you have a few clients, word will spread and you'll soon start getting referrals. It's always the first few clients that are the hardest to get.