Tanith I don't suppose your bites are on bits of the body where you have tight clothing when in the garden, are they (sockline; knickerline, both leg and waist; bra-line and straps etc.)? I get badly bitten by harvest mites (mentioned earlier by Tricia) at this time of year. The bites are almost all in those areas, or in other cosy intimate places such as under my breasts! Does that ring any bells with you?
I spent 2 years trying to work out what was biting me, and to this day, eight years from when it first started, have still never spotted the culprit, and treated the whole house and the beds before my research on the internet led me to the conclusion that it is harvest mites. The bites usually appear the day after I have been gardening, but by changing my clothes as soon as I have finished gardening has drastically reduced the number of bites I get.