Need legal advice about a bad employer?

i had a bad leg injury and was off for 18 months. i returned to work 6 months ago on a graded return and have struggled with the work. i agreed to work 4 8 to 10 hr shifts per week with the condition i had a break every 2 hrs (a shift that they have an abundance of) and after a while they started increasing the work load without consulting me. eventually i had a bit of a flair up with my knee and seen my GP who give me a note to my employer stating that i am to do a maximum of 32 hrs a week with 4 eight hr shifts and a break every 2 hrs. the note was for an initial 6 weeks and on my 3rd week they have chosen to ignore my GP's advice and increase my work load again despite the fact that they could easily accomodate the request. is there anything i can do about this ? p.s my job involves repetative use of the leg and climbing ladders