Hi cdodds2004,
I personally went down the estate agents route when the company i was subbing to told me they had no work for me with two days notice. I had a few jobs knocking around but def not enough to live on. Within two weeks i was inundated with work.
All i did was draw up a folder pack with your gas safe details, public liability insurance,price list etc etc. And went round all my letting agents within half an hour drive of my house. Don't bother sending letters or emails to people you have never spoke to as they go straight in the bin. The best thing to do is go in to the branch and ask for either the manager or whoever deals with the property maintenance. I literally struck gold doing this. Make sure whoever you talk to actually deals with gas work and your not just talking to the saturday boy who will take your card then bin it as soon as you have walked out the door.
Only drawback was i had to go drastically cheaper than any one else locally (£45 landlords cert inc service, £60 call out charge inc first hours labour). All i did was ring up loads of local competitors and find out what they were charging and undercut them all. And yes, agents can take a while to pay sometimes but as long as you keep on top with what they owe you and remind them of unpaid bills you will get paid.I personally have never had too much of a problem. If you do start working for agents find out what their charges are, as they normally charge 10% plus vat for the job, and add to your quote. I was caught out with this a couple of times.
The bulk of my work comes through letting agents and is mainly landlords certs and repairs but i have had boiler changes, full heating systems, powerflushes. Lots of nice jobs.And now i am their regular contractor i get asked to sort electrical faults, roofing and building work so i just sub it out to other contractors.
In my opinion it is the way to go. Might not be the way for you but it certainly worked for me.
cheers