Thanks 030675 for your tips. In fact I went back to reading the guide book (Burton . p 128) and discovered I had earlier missed his quoted phone number of the Tourist Info office at Silloth.
These helpful folk duly explained that on 12th April the high tide will occur in the morning, around 10 to 11am, and will be a mere 6metre one. Its only the extreme Spring and Autumn 8 metre high tides that cause flooding on the HW path.
I will try and get hold of their full 2010 local Tide Tables when up there and identify the dates when this might happen, then post it here for future paddlers.