I'd say that your £50 would be better spent on business literature that should be handed out to anyone you know personally as well as to all clients.
Make sure the web and email addresses go on all letterheads and business cards. If you currently have a large stock of literature without your URL printed on it, get a bunch of compliment slips printed off and include one with every clients paperwork and hand one over to every new client when a service is requested.
Make (or have made) a sign for the business premises with the web and email addresses on it/them.
If the business advertises in a local paper, include the URL/email address there too.
About the best way of getting the site known is the above methods.
£50 every few months is not worth wasting on web advertising in my opinion.
If you have well formed meta tags and keyword descriptions in your page headers (the ones you have aren't too bad) coupled with good descriptive text copy on your pages then sooner or later you will get spidered by the big search engines anyway.
You can manually submit your site to most of the big engines, but it can take months before things start to kick in to the point where your URL would show up in any search.
Think of it like this: how many solicitors are in your area, how many of them are online, how many do you think are in that position nationally and how the heck can you make sure that your site climbs above all of theirs ?
The simple answer is tyhat you can't without investing very substantial amounts of money, either for a professional engine submission and/or a Search Engine Optimisation rework of your site. Neither method (despite what some people might have you believe) is any guarantee of high rankings. Rather, they will help get you up there to an extent, but they cannot keep you there without further investment and is it really worth it in your circumstances for the sort of business and services we are talking about ?
Since that is hurdle you have to overcome, you either have to risk substantial amounts of cash for a paid for search engines submission scheme (with no guarantee of any results at all) or keep plugging away with company literature, local advertising, office window signs and manual search engine submissions and you will slowly build a well spidered site.
There are a few tricks and tweaks which can help optimise your site for the engines, but I'd suggest that the best way forward in your situation is just manual work to generate site requests. Nothing helps get a site spidered quite like getting hits from people typing in your URL and they will never do that by just trying to find a local solicitor online.
If you want to find a solicitor, you open the yellow pages and look for one. Even when you get spidered and ranked, unless someone searches for the business by name and/or township you are unlikely to be in the top ranks.
PurplePenny often suggests the DMOZ open directory project click here and you may want to consider that as an option.
It's not all doom and gloom, but you have to keep in mind that a solicitor is not the kind of thing people search the web for on a regular basis comapred to how often they will pick up the telephone book or call into the office when they need your services. The website should start by playing a supporting role to the business which, over time, will start to generate or assist in generating work.
Long term the site should earn its keep and more. Short term, you'd be wasting your money (in my opinion) by throwing £50 or so into web advertising every few months.
Just my two penneth.
Oh, one more thing. Run some searches in Google for:
"Search Engine Optimisation"
"Search Engine Optimization"
for lots of useful little tricks that on their own do little, but combined can help a great deal.
Hope this helps.