RAB Sensor Fibre string. Thanks to Keir for the tip! He writes in to say:
It’s a 1.27mm multifilament which gives high comfort and power similar to Tecnifibre 305, but without the fragility of the Tecnifibre string in cold weather. As used by: Sarah Jane Perry, Laura Massaro and many others! It’s very popular here in the UK.
It comes in a few different colors:
Image via racquetdepot.co.uk →