I've got a quote to add Solar Thermal to the current job of installing ASHP and Solar PV. As I've already got the solar cylinder, and the roofers and electrician are there anyway, they've given me a "good" price to add to Solar Thermal panels : £2500 for 2 x Nibe FP215, bits, and labour.
Looking online people seem to be paying about £4k for this system, but that includes a cylinder change (about £1k) and more plumbing work.