MC says we've spent 450,000 under the salary cap so what happens to the money are we allowed to keep it & spend the money on new players for next season or can we just bank it.

What he is saying which is pretty valid is:

If we spend £400k less than the cap
Could we save £400k next year and spend £800 under the cap and then the year after use the money to spend full cap.

I think the rules would allow it but you'd be risking being so out of your depth in Super League that you end up in MPG and then you'll be in a lower league and can't spend the money anyway.

If you survived on a smaller squad you could in theory bring in a big name marguee player the year after but it would be short term thinking.

The principle of not spending your money all at once though does work.
Carter himself has saved money to allow for a loan signing later if you need it. If you spend all your money on first day of season (or allocate it to be more exact) you have to sell or terminate contracts to change the team.

Surely we don't spend the full allowance because we need the money to finance many other things. The amount we spend on players is only the surplus after other essential expenses. We don't "save"anything.

Correct

Which is why we need more paying supporters.

And/or sponsors