Simple I thought, just unscrew the collar, lift it out and clean and grease the O-rings.
But lifting it out is a problem. It was quite a struggle to get it to move up at all, it's only come part way up and feels most reluctant to move any further.
The parts diagram at http://www.central-servicesuk.co.uk/shop/section.php?xSecw&xPage=1&jssCart efd1bb31fd15dd01f8f3c3b2bf834c <http://preview.tinyurl.com/orkujnn suggests it should be straightforward and there's no sign of any extra grub screw anywhere. Does anyone know if it's just a case of brute force or is there some secret knack?