Thanks Sarenco.
Allow me clarify my first question.
I am aware of the percentage annual limits based on age.
However, I am fairly certain that the following example is valid;
Year 2015: I can maximise tax relief by contributing 10k to pension (however, because I have some spare cash , I actually contribute 15k)
Year 2016: I have no spare cash this year BUT my overpayment of 5k in 2015 will mean I get tax relief on this 5k.
I am wondering how many years this would be permissable for, if I paid the 100K.