The Open University and the University of Wales Trinity St David's both do distance learning MAs.
Does anyone know which has the best quality teaching?

Difficult to tell expect one had previously studied or taught on both programmes. Nonetheless, having taken independent modules (not philosophy) from both unis, I think course materials from which module tutors teach are equally solid. That said, note that Lampeter enables you to follow either the MA in Philosophy or MA Applied Philosophy which in itself seems attractive to me. Both programmes offers a wide range of modules from which you take six and then write a dissertation. The OU programme leaves you with no options. You take two compulsory modules (which arguably may cover materials spread out in Lampeter's several modules) and do a dissertation. The problem with this is that, it doesn't allow you to follow specific modules which meet your specific learning needs. Another major difference is that whilst Lampeter's modules are examined by coursework (5000 essays) which could be developed into journal articles, OU modules are examined (I think) by TMAs and examination. Personally, I don't find postgraduate written examinations attractive at all. I'll rather write essays than write an exam.