I've made a little progress. I stripped down the 2150 to get a look at the polarizers. I marked the positions very carefully before removing them (both in and out on each colour) and cleaned them.
They all appear to be in excellent condition so the yellowing is coming from somewhere else. I cleaned as much as I could find with lint free cloth and compressed air and part of the yellowing is gone at the left hand side of the image. I feel confidence enough to strip the machine down a little further to try to clean all of the dichroic mirrors, mirrors and other reflectors. The biggest difficulty is with the optical engine. I can see that there are several filters etc which are very dusty but hard to get to . I can get back in there with compressed air and try to make a better job.
If it would help I can post pictures next time I dismantle the machine (probably the middle of next week) ?
Can anybody recommend any other cleaning methods. Would DSLR sensor cleaning fluid dabbed on a cloth ruin more than it would clean ?