My DD is in Y6 and has therefore finished her 11 plus exams. It sounds like our circumstances are very similar to yours when you say:
"Not a grammar area so a high number of pupils going for the small number of selective state (as well as wide assortment of private) schools"
We're in the West Midlands. DD goes to an average, small state primary, and has always been a ‘top group’ sort of child, with a particular strength in reading/English, but neither of us grew up in a grammar area so it was all new to us. After doing some research, we decided not to pay for any external tutoring, but instead to do some work with her at home using workbooks and a couple of mock tests (which I agree with a PP were extremely valuable). Our local comprehensive school is a good one, so a grammar place was not the be-all and end-all.
I would recommend that you make sure you know as much as you can about the style and format of the test(s) your DD will be taking. Over here they are CEM tests, and the focus is on speed, and they are apparently trying to test a wide vocabulary, rather than specific techniques that you can prepare for. The tests are designed to discourage tutoring as there are no past papers and they don’t give a lot of clues as to what will be covered. However, you can make sure your DD is solid with all aspects of the curriculum, and then work on problem solving, and gradually speed up.
4-5 hours per week of work specific to the 11 plus sounds like overkill to me. In Y5 DD spent about 15-20 minutes most school mornings prior to school, so certainly no more than 2h per week, until about June onwards, where we added some sort of timed test at home every couple of weeks. To be honest, we tailed off a bit over the summer holidays (when friends were pushing and panicking) because we felt she seemed to have got far enough with the preparation.
We started with CGP practice books in Maths, Verbal Reasoning and Non-Verbal Reasoning, and then moved on to 10-minute tests or other workbooks from other providers (Bond, Collins, First Past the Post, Letts, Schofield & Sims spring to mind) - to get a range of different questions from different providers.
For the CEM tests, I would say that what was more important than any of this preparation was READING. My DD has always been an avid reader, and I have encouraged her to read a range of books, including some classics and 'stretch' books. She reads out loud to us for a few minutes, a few times per week, and we talk about what is going on, looking up any new vocabulary. We also try to use wider vocabulary in conversations as a family (eg asking whether there is a more suitable word), and we enjoy the idiosyncrasies of the English language. Some of our friends had bright children that weren’t keen readers, and they had word lists stuck up around the house, but it’s not a very effective way to build up an excellent understanding of the written word.