Les
Rutty is correct that you can't use a stand but its also depending on the SH1 degree of classification as you are also limited to the height of the back rest, SH1A for example is no back rest on the chair.
You will have to do an assessment to get officialy catogorised and then get an exemption cert.

Although I have no experience of this other than recently we had disabled shooters who wished to shoot in the postal county team, ours were not eligable as they were SH2, under NSRA rules only SH1A is allowed to shoot against and with able bodied shooters in postal and live shoots.
Best of Luck,
Good shooting
Robin