The message
from "Roger Mills" contains these words:

No tank-jacket? (insert smiley of choice here)

Oh yes there is! The cylinder in encased in a factory-fitted foam jacket -
that's what the green stuff is, rather than copper corrosion! g

Yes, sorry, I meant no /extra/ tank jacket. The factory fitted stuff is
pretty poor.
From experience you'll find you lose a lot less heat if you add a
"traditional" jacket over the top. Ours was like yours when we moved in,
and a tank would stay hot for a day or so if we went away and turned the
hot water off. Now, with a second jacket and lagging on the neighbouring
pipes it's still noticeably warm after nearly a week.

--
Skipweasel
Pay no attention to that man behind the curtain.