I always make my own and it works brilliantly even on Horsefly.
Get 6 teabags pour a litre of boiling water over them and leave to go cold,
Get a 10ml bottle of citronella (ebay sell it cheap),
A 500ml bottle of malt vinegar (about 17p).
Put it all into a 2 litre coke bottle.Buy a spray bottle (ebay sell cheap) and GENTLY shake then pour into spray bottle as and when.
This works well for both flies and midges - several on the forum use it and have said it works.
5 caps of dettol - from a 500ml size bottle
250 ml of Avon Skin so Soft and fresh - 500 ml bath oil. This can be halved if midges not too bad.
1 teaspoon of citronella oil - can go up to 1 ½ if midges bad.
1 teaspoon of eucalyptus oil
1 teaspoon of lavender oil
1 teaspoon of tea tree oil
1 teaspoon cedarwood oil
Put into a 1 litre spray bottle. Top up to make 1 litre with either water or cold tea. Shake well before spraying and give a quick shake after every few squirts to make sure the oil is mixed with the water.
Spray on twice a day using enough to leave a fine mist on the coat. Ok for round sensitive areas.
I have used this on people as well as horses, but please do a patch test first in case you or your horse reacts to any of the above ingredients.
Cost works out about £3.00 a litre depending on whether you get the Avon on offer - not as cheap as some receipes but this does work well and smells nice.
Have started using on my horse who has sweet itch and is working very well! He wears a boett when not ridden - but i use the spray for riding and also put on his legs etc in the boett - for extra protection.
The one I make up does not, in fact it has the advantage that you have a nice glossy coat as a result of the essential oils/avon. Chancer gets a bath every two weeks (he is shown) and Cairo gets a bath a couple of times a year - neither one is showing more of an oily coat than the other. I flick then over with a brush and the shine really comes up.
If you don't have sweet itch or midge problems, then use lesser amount of the avon but don't leave it completly out as it really does make a difference to the effectiveness of the spray. Also remember to keep shaking when spraying to keep the oils mixed.