I've never worked in the UK, but the UK-based Association of C and C++ Users has a mailing list specifically meant for developers who want to introduce themselves to potential clients.
The accu-general is open to everyone, but to post to the above list I think you need to be a dues-paying member. The ACCU is a great organization, though, so it's well worth it to pay the dues.
If you are careful about it, Google Adwords is a great way to find new clients. The key is to bid on queries that only potential paying clients are likely to be searching for.
Don't bid on excessively general queries: I blew three hundred dollars per month for several months bidding on "Software Consultant" and "Software Connsulting", and got lots of clicks, but no inquiries from potential clients.