Experiment first Bill - the pigments and oils in modelling enamels are of a much finer grade and lighter viscosity than 'commercial' enamels. It should work, but it's just possible that LT might cause separation of the pigments.
One way to check if the paint itself is OK, as opposed to being faulty (or just plain cr@p, as most current Humbrol is these day) is to stir as normal, load a paint brush (medium sized, probably No.3) and transfer this to a pallete, such as a coffee jar lid, give it a quick 'swish' around, without thinner, then paint onto some scrap, laying off the paint properly. If the paint is OK, it should start to dry almost immediately, and be touch dry (but not fully safe to handle) in under an hour.
If it isn't - then bin it!