personally i would stick with blamphayne,they have what you need!!!! go in and see them, if theyre the only quality sawmill in your area then them being busy is understandable, old fashioned creosote is now illegal, youd be getting 'creocote' which as far as i can see isnt as good..
tanalised timber is safe for animals. and its time proved that it lasts, i cant imagine any firm offering and then honouring a 15yr guarantee, it wouldnt make sense, how would they quantify the conditions of use?
i think most wood thats been tanalised would be fit for purpose, the extra quality of redwood would be a bonus. truro sawmills would be a good call, but im not sure if your too far away from them. god, at this rate ill deliver it myself!!!
you wont find chestnut, its not grown as a crop like in france.
well thats my honest opinion, and i hope it saves you some money longterm.