alternating hot and cold water

We have an original Gledhill boilermate Model 200 it is getting on a bit (18 years) but has been the same for the 6 years we have lived here. The shower only stays hot for about 5 minutes and the only way to have a bath is to run a fine stream.
Is there a cure for this, or is the only solution a replacement by a 'tank in the loft' conventional system?
British Gas seem unable to solve the problem

I have a similar problem with an original Boilermate......20 years old! Any ideas or solutions to the problem please let me know

apparently it could be the sensors or the plate heat exchange we are hoping to get it sorted soon so will write again if successful.

Hi,
A BoilerMate II does not have a plate heat exchanger. A BM2 has a coil inside the main thermal store vessel. This coil is used to heat up cold mains water. This water becomes hot and then feeds through a Brawa-Mix valve which (when working correctly!) controls the temperature of the hot water giving DHW to all hot taps.
A BoilerMate 2000 does have a plate heat exchanger and the sensors to which you refer. Visit the Gledhill website and download the appropriate manual for more information.
Robin