Tricky one - looks like you are dealing with an old 106 that doesn't give you what you want (SO instead of rented units) as the 106 has been signed (?) this is clearly difficult to get away from - and I assume there is a new 106 / variation to the 106 round the corner which will cover the commuted sum?
It has been a whie since I worked for a LA enabling team (jumped ship to the RSL side) but would assume that a formulaic approach has to relate to build costs, on costs and sale values - clearly building on a heavily contaminated site in one town against a greenfeld site in the same town (assuming OMV's are the same) will be more expensive and therfore it is likely (very generally) that communted sums etc will be reduced on the brownfield site. However bear in mind that if the contaminated land was valued/bought for a third of the greenfield site this arguement doesn't work.
Don't the London boroughs have their very good model for assessing developer contributions both in terms of % and type of affordable housing and I presume commuted sums as well?
Alternatively I used to take the view that if a site was expected to deliver 10 social rented units without grant then I would expect the grant for 10 rented units to form the basis of the commuted sum for off site provision.
Although this is fraught with difficulties (free or open market land for the grant and the time delay for payment and what is the grant needed these days?) it is an option that get away from the land vales/OMV issues (although most developers will of course claim they have the highest build costs and lowest OMV's so can't possibly afford to do this!)
Best of luck.