Ford Iveco Horsebox - female driver needs help :0)?

I have a 7.5 ton Horsebox J plate, hasn't been used regular for about 6 months. Due for an MOT tomorrow, however when I went to get some diesel the lorry was revving really high when driving and I was having to put my foot on the brake to slow the lorry down. When I changed up or down in gear the revvs dropped back to normal and I was able to put my foot on the accelerator, but then the revvs would creap up again, luckily no horses where in the back because I felt a little out of control. Being a woman with a HGV, I just wondered if anyone knew what the problem could be, I want to make sure that I have some idea so I know what I should be paying for it to be fixed. Thanks for all your help in advance. Ps I've anyone knows of any HGV mechanics, reliable and honest in the Herts/Essex area then please let me know