An update at last ! It has come second hand but from a reliable source! Dad and Dave are now at Surat Thani ... and guessing by the text I've just received they've not long ago arrived. In fact they are probably still sitting on the station!
Their plan was to catch a train today down through Thailand. The first one they tried had no compartment for the bikes, the second one they tried was full! So they hung around all day and they eventually got on a train at 4.30 pm (local time). It was due to arrive at Surat Thani station at 2.00 am. Not sure how they'd go on trying to find any accommodation at 2.00 am in the morning! I got a text off Dave at 10.00 pm (GMT) which would be 5.00 am Thai time to say they were at the station and the reason Dad hadn't text me was he'd let his phone battery go flat! I doubt he'll find anywhere charge a phone on a railway station!
They've managed to cover quite a bit of Thailand, but still got some miles to cover to get back to Phuket. Their flight home is Saturday evening so they got a couple of days of pedalling to go.
Will keep you posted.