I’m currently working on  an invention in the mains trailing extension space. I’ve had some useful advice including a free patent search carried out by the IP centre at the British Library. (Be aware that the re-searcher’s time is free but access to the Derwent database is chargeable. There are also constraints on qualification for free searching , so best to check first with the IP centre.)  http://www.bl.uk/bipc/ for more details.