large lumps caused by cleg fly ?

Sounds like its fly bites to me. Bl**dy horse flies are terrible at the moment. You may have to speak to your vet about antihistamines?
 
My mare gets this sort of reaction to bites too. I don't actually treat the bites once they have happened, but to prevent her getting bitten in the first place I have found Leovet Power Phaser to be the most effective repellent.
 
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