I believe the Glow Worm Space Saver Mk 2 boilers were last made in 1988 -
http://www.homeheatingguide.co.uk/efficiency-tables.php
The efficiency rating is shown as 65% which is very low in relation to the 90% of modern boilers.
The immersion heater is not an indication of anything sinister - the system requires a hot water storage tank (combi boilers do not have this luxury) and it's a sensible precaution to have an immersion heater in the tank to cater for any boiler faults - I have an "immersion" and have used it for hot water twice in 5 years - almost a life saver.
I'm not sure if the energy rating in the seller's documents take the boiler efficiency into account - I can imagine a certain lack of insulation and double glazing contributing to the low score.
If you plan to buy the place and install a replacement boiler expect to pay in excess of £2500 unless you go with British Gas in which case it will be considerably in excess of £2500.
Good luck...
Peccavi
July 2009