I'd say the odds are better than evens that your Agency has been skimming £35 per day off your rate since you started the contract. Any reason which includes "the client wants to pay less because... [fill in any nonsensical reason which you can't easily check HERE] usually means they are.
I'd ask the client what they are paying the Agency and what %age mark-up is included. Be honest and tell them your concern re: rate skimming. The fact you are being renewed again tells you that the client is happy; hence you have plenty of leverage to find out the truth.
I could be wrong of course, and this particular Agency may turn out to be completely honest... and I'm the Queen of Sheba

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