(I promise I have no connection, although it's my second recommendation of the site in a week. I'm just a bit of a nut about things being serviced properly after my uni friends were nearly killed by carbon monoxide from a dodgy gas heater.)
I use a company called Serviceteam who are based in central London. They did major electric works in my old flat, sorted out the mess that another company made installing a boiler (A-Class) and I have just had then renovate my new place including installation of a new boiler. I can't fault them. No connections, just a happy customer.
Not sure of the cost though. I use them for everything now as I know they deliver.
I needed a landlord's gas safety certificate in the summer and started by looking up local engineers on the gas safe website (as per the second post) but to be honest found them quite expensive. Up until then, I had just used British Gas.
I then found these guys online(www.landlord-certificates.co.uk) who were much cheaper and also Gas Safe registered. I think it was about £55 for a certificate and service. The guy turned up on time (and actually rang ahead as requested) and went about his job with minimal fuss. I think the company uses freelance engineers.