The problem I have is I need a 12 month travel card, what I need to know is can you get a 12 month season ticket from Leatherhead to the Zone 6 boundary or do I have to get it to a station within zone 6 (Ewell West).
Unfortunately Season tickets are not available to "boundary zones".
Buying a Ewell West - Leatherhead Season would be fine, but you would have to travel on trains which stop at Ewell West (not sure if this is a problem; I think most or all do but I'm not familiar with the area to be sure).
I would think the best solution would be to refund your annual Z1-6 Travelcard and purchase a Leatherhead - Zones 1-6 annual season (£2580). It
might be the case that a combination of tickets is cheaper, but it's usually not so.
Edit:
Leatherhead-Z1-6 = £2580
Z1-6 only = £2016
Ewell West - Leatherhead = £940
Ewell West - Z5-6 (closest to a "boundary zone" season) = £1300
Thus buying separate tickets is significantly more expensive.
BTW - these prices go up on 2nd January (as I expect you realise!) so buy tickets to start before then to avoid the price increases.