I don't think that Bulbophyllums are any more susceptible to viruses than other orchids. The only way to be certain (either positive or negative) is to test. I don't know the availability of test kits in the UK... there is a company in Taiwan,
Rega that sells these and ships internationally. But their minimum order is 50. So a significant investment. One can't tell if a plant is virused by looking, but if a plant is infected, it is likely weakened. So if a plant is not thriving, that could be one indicator but there are many other reasons for a plant to not thrive. Other diseases, insects/mites, or cultural problems... an endless list. My advice would be to simply observe your orchids. If you have one that isn't thriving, separate it from the others. Whether it's disease or infestation of any sort, you don't want to spread it. Utilize good hygiene when repotting.(Sterile tools, clean surfaces, hand washing or gloves, etc) And then don't worry about it. If your plants look healthy, they probably are.