The message
from "Mary Fisher" contains these words:

Ours cost us £2000 two years ago. Our gas bills (the only other
water heating we had) were reduced by almost £300 in the first year.

There was a salesman extolling the virtues of solar heating on BBC
television news this am. Claimed installition costs was in the region
£3500 - £5000 and a saving of 10 - 15% of heating bills could be
expected. On that basis I expect Mary's gas bill would have been in the
region of £2000 - £3000 prior to the installation of solar heating.

--
Roger Chapman