The healthy house sell a range of masks, although not sure you can wear them to bed, you would need to check on this:www.healthy-house.co.uk
0845 450 5950
You mentioned you have you have the bedding covers, an air purifyer in your bedroom may help you, as it filters the air & the fine meshing system will then prevent the allergen from coming back into the room.http://www.allergyuk.org/prod_soa_air.aspx
You did not mention if you are taking a daily antihistamine along with nasal sprays, both can be on prescription from the GP. You may also find that a nasal irrigation system such as Neilmed sinus rinse or Sterimar, which are both available from Boots or Lloyds pharmacy, will help with the nasal congestion.
Please take a look at our factsheet, avoiding indoor allergens for helpful tips that may help:http://www.allergyuk.org/fs_indoor.aspx
Hopefully this will help help you, but if not it would probably be a good idea for your GP to refer you to see the allergist again.
All the best