Thanks for all the advice guys.
Everybody seems to be saying that I need to increase the quantity of running. Clearly the more time on my feet, the better.
I have adjusted my programme to include the "semi-long" run, which seems to make good sense, which I plan to build up to 12 miles. I will try to do some "speedier" miles as part of that run.
My plan now has my weekly mileage peaking at just under 50mpw, previously I only built up to about 35, so hopefully taking on board all the comments above, that should make a big difference.
I'm still not sure about fitting in 6 sessions per week, it may be difficult for me practically, although there is always early mornings, but I must confess, I'm pretty crap at getting up for early morning runs!!
I'm still not sure I can fit in all the variations on pace suggested by Mathias, I think with limited time, the concensus would be practise at marathon pace as much as possible? Although tbh, I'm not quite sure what pace that would be!!!
If I look at the McMillan pace calculator, going off my 1/2mara time, would give a predicted mara pace of 9:38 min miles, but my 10k time would equate to 9:24. When I did my first marathon in May (when I was disappointed with the time), the pace was 10:45 - Which is slower that I am doing my LSRs???
I know the calculator is only a guide etc etc. but if I want to try to build in some miles at mara pace - what pace would that be???
Sorry to ramble on!! My gut feeling is to try to work off my 1/2 time, as I have done several races at around virtually the same time, but I am wary that you should train to your actual fitness level, not a "wish list".