You know, James, you should keep a copy of the time sheet you have sent in. If you physically give them to someone, get them to sign and date both your copy and their copy to say that they have received it. If it is sent via the post, then pay that little bit more and ensure there is a signature for it when it arrives. When they say it has not been received you can trace it and find out who signed for it.
My next bit of advice is to contact your local Citizens Advice Bureau, or your local Tribunal Office and discus it with them. They are in an extremely good position to advise you, and help you take whatever action is deemed necessary to rectify these problems with your company.
You could also quote the Discrimination Act at your managers.....but most importantly keep a record of everything that is said, hinted at, and done to you...it will all work as proof against your employers.
Work won't like it when they realise that not only will they be accused of forcing you out of a job, but they will also have to pay a huge amount of compensation, court costs, and a massive fine for going against the Discrimination Act.
Wish I could be kept posted on this....I could use it as an example in my Diabetes Awareness talks.