How do I bathroom / kitchen / extension? etc.

Sorry to hear about your problems @Sparky

It does sound like the cost of the main contractor is the main issue here, and the situation would be a lot better if you could find a cheaper one. But for relations to break down so early with your architect is obviously not great.

In my experience chemistry and sharing a vision is incredibly important when you use an architect, and realistically if things are falling apart so early that could be a red flag.

Obviously @hoefla knows what she's talking about but I particularly agree with this bit:

In current climate I'm surprised any architect is offering assurances on costs, it's not under their control.