Mrs Steptoe,
Are you sure you are not being defeatist? Although that's the pot calling the kettle grey - I am in exactly the same situation and having nightmares about plan C and plan D, etc. I don't know from what experience your English tutor speaks, but St Benedicts is really not that selective - if his teachers are saying he is high ability, I can't imagine he won't get in there. Re practice papers, my daughter is also very uneven - but I think it's also a bit of a burn out, when they do practice papers every week at school, and then at home, I don't think they concentrate and care that much. I stopped doing practice papers for now and am working on specific areas instead. E.g., for maths, I cut up all the papers into separate topics/task and have different folders with assorted problems from various papers but on the same topic. Once we go through these, I will start giving papers again, but after a bit of a break.
I would try to concentrate on 11+ for now rather than looking for a prep - because if you are not considering boarding, then you will have this nightmare all over again with 13+ day entry in 1.5 years. Do you really want to go through this again?
BTW, what sort of scores he gets on practice papers and what are the papers? Who grades them? I think if he gets 55-60% on Dulwich papers, he should definitely get into St Benedicts or Emanuel.
Also, I think you may be over pessimistic - SW/SE London is quite well served in terms of boys schools, I feel much better than the girls, i.e. you have Dulwich, Trinity, Whitgift, Eltham college plus co-eds like Emanuel, Ibstock, Harrodian, Kingston Grammar, Latymer Upper. Hampton further afield. My point is that people will be applying to 4/5 but can only go to 1! So there is more hope than it seems on the surface. Well, at least that's what I keep telling myself when I am plunged into the depths of despair by OUR practice papers.
In terms of preps, there is also Thomas's and Eaton House. I am sure they will have movement as some boys will.
Also have you looked at Claremont Fan Court - two boys of my colleague's sisters go there and are happy, but I think it really is Plan C/D.