Vents

We don't spray it inside the house - but on window frames, door frames, and 'ant pathways' on the outside the house.  This stops them in their tracks without affecting us at all.  However, as a bonus, we've discovered that it also prevents flies wanting to come in, too!

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Thats great many thanks will give it a try

 Could well be these little darlings. Will give them a suck with the hoover and persuade them to cluster somewhere else. many thanks

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Thanks to everyone for the great advise. Went to the caravan at the weekend and the weather was great so opened the sky light and decided to have a look out of it. The roof is greener than my lawn at home. Having it valeted this week so hopefully this will solve most of the problems.

Nothing  wrong  with  having  it  valeted,  Kitchy1  just  keep  the  Hi-Pressure  hoses  away  !!  They  are  the  fastest  way  to  remove  sealant  from  round  doors,  windows  etc  known  to  Man ( OR  Woman  come  to  that  !!)

Cluster flies are the main culprit in many areas. They seek out cosy places to hibenate in Autumn and vans are ideal. One will find a way in and then the rest follow. Interesting life cycle to these little beasties! 

A couple of years back my van was 'hibernation central' for these flies whilst in storage. When we took it out at Christmas once the heating was on they emerged one by one and then, during the next few days by the dozen! A right nightmare.

https://www.rentokil.co.uk/blog/the-strange-life-of-cluster-flies/#.WeUJy-t4WrU

Also, these cluster flies are attracted to warm vehicle engines in rural areas during early/mid summer when they are looking to lay their eggs on unsuspecting earth worms. What appears to be an infested area on arrival at a site soon becomes clear as the vehicle engine cools down. They then 'disappear'.