I would ignore the HV Becky....a lot of them are useless
I would take photos, notes, symptom lists and your thoughts on what you want and go to the GP and see what the GP has to say....it isn't really a thing that the HV has any jurisdiction over and just because the HV does not take you seriously means nothing at all.
One thing to keep in mind is that at 8 months, it generally has to be pretty severe or they have to have a reason to worry about allergies (rather than intolerances iyswim) to test a baby of that age. ie/ worry about anaphalyctic shock, and so on. If it is "just" eczema then really they will want to do things by elimination and trials first - elimination diet if you suspect foods and removing things like perfumes, dyes, soaps, etc from laundry and so on.