Been investigating the CH system (pretty old pumped CH/Gravity HW)
before I repair/upgrade it.

This device is in the CH flow pipe:

http://familyfrench.co.uk/temp/IMG_6761.jpg

For reference the pipe it is in is about 44 mm diameter (1 3/4 inch?)

The yellow label - along with the makers name and a BS mark says:

10.5/l 86 C (as in centigrade)

What is it?
--
Chris French