tomdav, I'm as wise as you are but I would imagine that whether your UMS marks are high enough will be judged in relation to the calibre of the other applicants in your year.
While the fact Cambridge asks for the modular UMS marks suggests that they are used as a tool for judging applicants, remember that many other factors are also important (such as your personal statement, interview performance, reference, predictions &c.) in deciding whether or not to make a candidate an offer.
The instructions for applicants information on the AS/A Level Unit Sheet says:
"If you are taking AS and A levels please complete this form so we know which units you have taken, which units you are currently studying towards and which units you are planning to retake. You should enter the UMS scores you have received for all completed units (both last year and this) below. Please note that UMS scores should be listed for each sitting of the unit and regardless of whether or not you have certificated or retaken the unit. Units should be listed by subject rather than by date taken. In addition please enter details of the units that you are currently studying towards/planning to take overleaf. If you are applying post-A level you should also record your unit marks in the appropriate section.
[We appreciate that in some schools/colleges early units are done only for practice and indicate little about eventual performance.]"