Do you have any flashing lights on the front of the boiler when the tap is on?
No, there are no flashing lights - either with or without the tap running
OK, then the boiler is not acknowledging a demand for hot water. This leads to the flow sensor again - possibly a blocked filter.
If the boiler actually fires for heating, and the flow sensor is working then you could have a dead PCB.
The boiler fires for heating.
What is a PCB? And is it difficult/expensive to repair?
New they costs about £270+VAT, although it varies from supplier to supplier. PCB stands for Printed Circuit Board.
You would need to add labour to that.
The boiler is only 2 years old. Is it normal for the PCB to go wrong so quickly? How long on average does it take to fix?
Less than an hour, but if the boiler is so new it would be worth contacting Glow worm about a warranty repair. In fairness, although I do not rate their boilers, electrical faults are note that common in my experience. However, modern electronics are susceptible to all sorts of external influences.
When was the flow sensor replaced? could water have been spilt over the electronics?
Flow switch was replaced less than 2 weeks ago.
No idea whether water was spilt. If it was, how could this be repaired?
How long (on average) should a combi boiler last? And is it fairly common to have to replace parts over it's lifetime?
Your level of boiler should last 10 - 12 years if looked after. Major components can go at any time, but I would take issue if you had to replace anything other than service items in the first 3 or 4 years (service items are the odd gasket and maybe flame probes).
If water was spilt on the board, then it will be goosed. If water was spilt with the power off and subsequently dried up before switching on, you have a decent chance of it working again - but I think that ship has sailed.
The Betacom doesn't enjoy the best reputation in the world, but with proper servicing (not by the likes of British Gas) and an installation done to the letter of the installation manuals, and all the normative documents referenced within; it should last. My advice is to look for a local specialist... Perhaps someone who has worked for Group Service (the people in charge of warranty repairs for Glow Worm and Vaillant boilers (same companies). Of course you need to get this problem sorted first.
For my customers, I monitor repair bills, and once a boiler has passed 5 or 6 years, no more than purchase price of the boiler should have been spent on anything other than annual services.