Si wrote:wonder if, in these cost cutting days, they are using something that melts more readily?
OK here's your answer, if you trust the BBC to report technical stuff accurate. (I don't usually but this seems simple enough them to get it right.)
Since 1995, the busiest roads get a more expensive tarmac which softens at 80C. (That's road temp, not air temp).
Other roads get a cheaper tarmac which softens at 50C.
About 5% of road length has been surfaced with the 80C stuff.
If you see something different, it was a mistake.
http://www.bbc.co.uk/news/magazine-23315384
However this is simplified because they haven't taken into account roads which might have had a surface dressing. When you get covered in tar it is usually a recently applied surface dressing that has melted.