Doesn't look rotten just needs a good clean and clear wax treatment people think decking is maintance free when not
But if you want too replace do it with solid composite deck replace batterns with composite too then just clean once a month an tower fine if ya don't like heights
Have you fixed a vent to the bottom battern if so remove battern an fix vent straight to brickwork as others have said you need 40 max no upstand for slates or you will be adding lathes untill you flatten out
As above on the squares centres for adjusting toe but you also need a cranking bar to adjust hinges left an right youo see these hinges on cheap door setts used in new builds normally
yes they do let you know when too much oil mine always does this after service let the engine run for 5 or more minutes then check but im lucky the 3l diesel has dip stick aswell
have you checked with your local council as they do release land for this
http://www.self-build.co.uk/right-to-build
https://www.gov.uk/government/news/boost-for-aspiring-self-builders