So I suppose that the idea of an allowance furthered the " motive " in Jeremy's supposed killings ?? 
Something else that the relatives et al could sink their teeth into.
I've found the relevant reference in Carol Ann Lee's book under the "Growth" section. It says June felt that as Sheila would turn 28 on 18 July she should become more independent by managing money herself and to this end a deed of covenant was drawn up by Basil Cock to the sum of £3000pa to be paid in four quarterly instalments.
Of course this does give further momentum to the argument that Jeremy killed his family for financial motives, as he already had access to the safe key ( the document was waiting for June's signature), there were the bills for Sheila's hospital stay in March 1985 on Nevill's desk totalling £15,000, and in addition Colin mentions in his book that Sheila would have liked the twins to attend private school.