Phil,
I think different rules must apply in Ireland as some tracks only have 15 fences in three mile chases (Roscommon, Down Royal & Wexford spring to mind).
Unless courses like Taunton (who have got round the 6 fence per mile rule by having races over 2miles 7.5 furlongs (17 fences)), I presume Market Rasen must have a legitimate excemption – presumably once the Water was removed, which I believe was to avoid a dog-leg run in on the chase course, there was no suitable site for a replacement fence, although I’m sure Lekha will put us straight.
Slightly off topic, but is the third last at Newton Abbot a water-jump, a dry "water jump" (a la Huntingdon) as I’m sure I heard Martin Harris refer to it as a "stretch-fence" over the weekend.