Have a Ravenheat cs80 boiler not getting any hot water or central heating and it is not showing any error codes when running hot water it makes like it is trying to work but water stays cold

SOURCE: gas central heating boiler. insuffient water pressure.

u need to have a key (its like a T shape key) and u put that under the boiler its a hole in there, than u open the tap which is near this hole.

SOURCE: Combi Boiler - Running warm & then cold!? HELP!

mixing valve must be replaced

SOURCE: COMBI BOILER /CENTRAL HEATING

the boiler should modulate to controll the radiator temperature but the central heating will go off whilst you are drawing hot water

SOURCE: combi cutting out and overheating

is this a baxi 105? chances are you diaphragm has gone on your diverter valve. Or your diverter valve.
Do you will initially get a liitle hot water if your heating is on?

SOURCE: valient combi boiler is not heating the water properly

vailant boilers are one of the best on the market and should last you more than 6 years depending on installation and servicing.
the engineer you had should not have replaced the diverter valve if the boiler was firing up when the taps are opened. if you was getting luke warm water with the boiler alight it is a classic symptom of a blocked plate heat exchanger on the central heating side.
a new heat exchanger can cost around £100 depending on the age and model of the boiler.alteranatively it could be descaled but may clogg again. it would be a good ideal to fit a magna clean filter on the system to help prevent further debris if you have a dirty system .if you don't trust your engineer i suggest you ask another. find out if he is registered first and don't pay until you are satisfied with the work. if you are asking the same engineer back try and get a discount off the work as he has failed to rectifiy the original fault and fitted the wrong part due to wrong diagnostics. if he is registered you can complain to the governing body ie corgi or gas safe register.
good luck.