I've misscarried twice before and have just found out I'm pregnant again but the doctor won't sign me off wrk as she said I'm pregnant not ill, I changed doctors thinking it was just this doctor but they still won't sign me off which I don't understand surely I'm at higher risk 2 misscarry again so why won't they sign me off!

It often depends on where you are, but generally you won't be considered high-risk until after 3 miscarriages or unless a specific medical reason can be determined for their cause that would make it necessary for you to be off work. This is because miscarriage is fairly common. Many women will never even know they had one as many happen before even knowing you are pregnant.
Getting signed off can also depend on what sort of job you do... obviously some are more of a risk than others.
So sorry to hear about your previous loses... wishing you well for this pregnancy!