Ariston Primo unvented cylinder

Hi,
Am a plumber based in Suffolk. In March this year was called into a new rental property to replace the thermostat pocket on a Ariston Primo ITI 210, as existing pocket had failed and was leaking. Bought part from an online retailer as local merchants didn't recognise Ariston part number. Been called back today as cylinder was leaking again...from the thermo pocket which had failed...after only 4months!
Anybody got any experience of this type of failure and what could be the root cause??
Thanks.

[AlanE]

How old is cylinder?

Cylinder was installed (not by me) in May '08. Original fault reported to Ariston in Feb '10 (again, not by me) and was going to be carried out under warranty. They tried for 3 weeks to contact the tenant to gain access without success, so they cancelled the job. Tenant complained to the letting agent that nothing was being done, so they got me me to carry out the repair. I contacted Ariston yesterday but as the cylinder is now out of warranty, they are not interested.
I left the faulty thermo pocket in a glass of water overnight, and this morning it was full of water, so is definately faulty. If I were Ariston, I would be interested to know why 2 of their componenets had failed.