It's possible that a 6 bedroom house in that area is just too large - what I mean is that it may have been 'over extended' in the past to suit a particular family. From the pictures, it does seem to have had a lot of extension work done. Possibly the annex was a granny annex and the layout now wouldn't suit everyone? It's not everyone who would want two kitchens in their property, for instance. So, youre looking at a very small, niche market.
On Rightmove there is another 6 bedroom property in Cliffsend on the market for £385,000. Have you looked at that one in comparison to yours? Why would you, or the agent (I think it's the same agent), think yours is worth £25,000 more?
Mouseprice says its been on the market for over 8 months and the price has been substantially reduced, is that right? To me (and we're actually looking to move house and we did initially consider Kent) it would make me slightly wary as I would immediately wonder what was wrong with it. Also, I'd obviously be looking at the cheaper 6 bedroom property in preference - unless we were particularly looking for a granny annex.
Sorry, not actually much help, just my observations.