We took off all of our vents, the floor and the ceiling and places fine bug mesh over them before replacing them. Not had a problem since and it was a fairly big issue before. Like Gerry said, you need to make sure you don't block your vents as they are there for air movement and to supply fresh air when burning gas.
I hope you get it sorted as it was driving me nuts and I just couldn't figure out how they were getting in. Now, happy out
Linkback: https://staticforum.co.uk/index.php/topic,711.msg1852.html#msg1852