Have I definitely got herpes if blisters on red patch?

Maybe. Very few conditions present as "blisters on a red patch", and most commonly it will be herpes. However, that "herpes" can be either caused by herpes simplex virus (hsv 1 or 2) that cause fever blisters and genital herpes, or by varicella zoster virus, which causes shingles. The latter will present usually as a painful rash.