A search for admissionlady's posts may help you. I think she posted about the reasoning behind the changes in the process.
If I were preparing for Sutton, I would take the following into account:-
a) the change in admission criteria: 75 places are reserved for boys in certain postcodes. Bear in mind that there are only around 1,100 year 6 boys in the entire borough of Sutton. Even if half of them sat the SET, and knowing that the SET had around 40% passing, you'd be looking at most at around 220 boys in Sutton who will be sitting the second round. The odds of getting a SGS place if you are local are not bad, as I suspect that far less than 50% of all year 6 boys sat the SET, let alone favour SGS.
b) I don't think the headmaster has changed recently. If not, then from this forum we know a) that he is a statistician, b) he personally reads all of the essays of the highest scoring boys who would be likely to gain admission, and places a lot of weight on the English and c) he knows the level of tests that he needs to set to get the boys he wants.
c) Sutton Grammar sits the WJEC English Language GCSE.
In light of the above I don't think that the Sutton maths will be as difficult as the other 2. Still challenging, but not quite as challenging as today. English - I would be expecting 2 written pieces, around a side of A4 each. Traditionally the SGS writing tasks have been persuasive writing and a traditional essay. I would certainly look at the foundation level WJEC English papers for ideas on the type of tasks which might be set, eg http://pastpapers.download.wjec.co.uk/a12-4172-01.pdf
Thank you very much LM. Very helpful as always. Got completely distracted by Wallington's Question4Gate saga and didn't reply to your post! Do update if you have any more new info.