Just got off the phone to HMRC and it seems that the Armed Forces Uniform Laundry Allowance (or whatever they are calling it) has been approved and applied already, as my new tax code proves.
Whilst that won't help with the latest SA, it can only be an improvement if the threshold goes up!
The lady on the phone said they hadn't had guidance yet as to whether it will get backdated but I'm not holding my breath on that one!!
Every little helps!
Thanks for all the other snippets of advice to those that replied.