Thank you all for you comments and advice. To answer a few questions in one,
I have a 180kw system which is registered under district heating as it feeds two houses, One very large victorian (single glazing, solid walls etc) and a converted stables in the garden. Both are in a conservation zone hence no double glazing etc so they are inefficient and require a lot of heat to keep them warm.
As for the 100tonne, that not all to go through my boilers, Some is for family member's stoves and wood burners. The deal is I buy the wood and equipment, they provide some free labour and we all get the wood we need. We're currently producing our own firewood already and we're just looking to scale things up and make it a bit more time and labour efficient. Also, as i'm sure your all aware 100 tonne 'wet' is more like 70 tonnes once seasoned. Thats how much will be burned.
THE WORK!! - I am aware its a big commitment and its going to be a lot of work. I have an office job through the week so believe it or not getting out in the woods and doing some 'proper work' is actually enjoyable.
The reason I'm thinking of planning for bigger timber is that with the amount of firewood processors about all looking for 12" logs they can be in short supply and therefore more expensive (I'm happy to be corrected on this). If I plan for larger logs from the off then I can take any timber thats going.
"why not buy from an established firewood trader?" Seasoned softwood sells for £150 tonne minimum (again, i'd love to be corrected). If I was to buy our 70 tonnes of seasoned in that would cost me £10500 as opposed to the £4000 to buy in and process ourselves. It may be a couple of weeks hard graft a year but for that sort of saving its worth it.
Back to the original question about chainsaws, lots of food for thought, thanks.