I use Piriton for my dog and buy it in pots of 500 from
here. £9.99. I find it works well during the pollen season but do check with your vet re combining with your other medication. The amount your dog can safely take will be dependant on her weight.
I also use a dilute solution of Hibiscrub to wash off mud after walks and wipe dust away from my boys eyes and mouth. This has made a noticable difference to his itch scratch itch cycle.
Dermacton is a very useful cream for any sore patches.
If you are on an immuno therapy regime with regular injections your dog shouldn't also be taking oral steroids - perhaps I've misunderstood. Even on injections we used to sometimes need a topical steroid gel but with a pro active and vigilant approach to any signs of redness or scratching, and using the above methods I can keep things quite calm and I no longer use any steroids.
HTH - itchy dogs can be very distressing