I was still unwell yesterday so went to the doctor who took some blood for tests. The results came through this morning and the diagnosis is that I have Dengue Fever, a disease spread by mosquitos and the symptoms of which in my case are muscle and joint pain, fever, fatigue, and headaches. It's present in most of South East Asia but especially in the islands of Thailand and especially in the months of March to July. My bad luck to be in the wrong place at the wrong time (though I may have got the mossie bite before I arrived in the islands). There's no treatment for it, just rest and paracetemol.
My main worry is that my Thai visa runs out at the end of the month. Unlike arrivals by air who get 30-day visas, arrivals by land only get 15 days. So once I'm feeling well I'll have to rush to the border to cross over to Cambodia.