I had intermmitant hot water problems with my 2002 linea 28. ie with shower and running baths and general washing up!, removed and descaled primary heat exchanger (only 2 posi screws with hex sides for your info) descaled with some brick acid i had and flushed out with clean water, lots of crap came out! and while it was removed it was simple to remove sensor and when removed it had plenty of scale (was 8 years old!) replaced for a tenner plus vat, built back up and water problem solved! no doubt saved myself a fortune on calling out an engineer!
Jon Holly
December 2010