As an experienced allotment grower and having had success with sprouts before you obviously know how to grow Brussels sprouts, but if it’s any consolation our sprouts at Stephanie’s Kitchen garden are very slow this year too.
Without seeing the sprouts and looking at the conditions around it’s difficult to advise but I don’t think the leek rust has any connection with your problem. It is important that you rotate your crops you didn't mention if you were practising crop rotation but if not this could be the problem.
I would do a soil test to see what your soil pH is, Brussels ideally need a firm soil with a ph of 6 – 7.5 – if you haven’t got this then the addition of lime will make the soil more alkaline.
The number of sprouts is dependant on the number of leaves. For plenty of leaves early sowing (late winter or early spring) is essential and careful planting into very fertile soil and subsequent careful watering is necessary.
They like a pre-prepared manured fertile soil and a feed with Growmore or Blood, Fish & Bone during the growing season will benefit the plants too. Water every 10-14 days in periods of dry weather. Plants benefit from a top-dressing of high nitrogen fertiliser such as dried poultry manure pellets at 150g per square metre in July (which you are doing) to encourage leaf growth and eventually more sprouts.
Often it is weather conditions that affect our crops and maybe our dry spring and wet and cool summer are to blame this year, but if you follow the guidelines above fingers crossed you should see a good crop next year.