We were up at Bob Browns Nursery, Cotswold Garden Flowers a couple of weeks ago, it was a two and a half hour trip each way, bought some more plants. Must be fairly near you Chrissie.
What time of year did you take the B. sanguinea cutting. Is the damage to the variegated Brugmansia just the leaves at the top of the plant or all over.
Red spider mite we get here on the Brugs, as you say nothing seems to get rid of them, have tried quite a few products, some of the products aim to suffocate them, but they leave the plant very sticky. I think the best bet is to hose the underneath of the leaves regularly, but here that can cause a problem to the plants underneath the Brugs, they end up with the spider mite, so you have to hose them as well.
When we bring the Brugs in for winter, we trim the branches back to leave the main structure, then we rub off all remaining leaves, then give the bare branches a hosing down to remove any remaining spider mite. Let them dry and bring in the conservatory.
They soon start to regrow leaves, so much so they hit the roof before Spring, so have to be trimmed back again. The conservatory is heated to +9 or 10 centigrade. Many other plants shelter there for the winter, so keeping down the bugs is important. Come the Spring the Brugs are hauled outside and they are taken out of their pots, 50% of the soil and fibrous roots are cut off with an old saw. They are then stood back in the same pots and refilled with new soil. Then taken back into the conservatory till we deem that all frost has ceased. Then back outside with them in a not too sunny position to get them used to the brighter light, otherwise the sun can damage the leaves. When they have got used to the outside they are put in there permanent positions, still in pots, for the summer. A lot of work, we have ten big standard Brugs. We trained them as standards so we can plant underneath them.
Malcolm