Asking this on behalf of my sister.
She recently had immersion heater replaced (council property) as hers had stopped working. She only uses it as back up as they have gas boiler for water. Since it has been replaced, the water doesn't stay hot that long which never happened before.
The chap who did the work cut a small section of the rim away at the top of the tank because it was a different element/fitting he was putting in! I personally think that the chap was wrong in cutting the tank as it has exposed some of the foam under the glass fibre. Am I right in thinking this has compromised the retention of hot water, as she's not sure if it's that or boiler needs attention! The link below shows the type of tank it is or near as dammit. Your advice would be appreciated please. Many thanks.
http://www.wickes.co.uk/invt/221228