I've already been in touch with Triton to tell them of my problem, and their initial suggestion was a locking nut. Anyone with a Triton router will tell you that a locking nut will severely limit the depth of cut possible. So I've replied, telling them of Neil's brilliantly simple suggestion of a coil spring under the thumbwheel, which I've tried. It works well, so all royalties will be channelled to Ireland.
And no, the spring from a ballpen won't work - the spring has to fit over a 5mm stud.
John