Ozzy was back at our usual vets for more general health blood tests on Tuesday - Steve says that he will ask for an 'elderly dog workup' (?) as well because although he isn't elderly, the diseases that can cause the lipid deposit are things that often can afflict an elderly dog, although not always. One of the vet nurses also managed to get a urine sample from him. I wouldn't know where to start, but apparently she took him to the area where they take all the dogs for a wee, and he had a good sniff then obligingly cocked his leg and she managed to catch it - amazing, she's my hero now! Should get any results today or tomorrow.
Update - just had a call from Steve the vet - all Ozzy's blood tests are clear, nothing abnormal at all which is a relief. However, still a little frustrating as it turns out that they can't find the cause of the lipid deposit, so subsequently there is no treatment, so he, and us, will have to just live with it. Apparently the cornea is clear, so there will only be minimal loss of vision, maybe a bit foggy, but he can certainly still see perfectly well and that is unlikely to change.