Limiting Shipping Options Depending on Payment Option selected

Hi,
I don't seem to be able to find the answer to this one.

I have various payment options and various shipping options defined.

One of my Shipping options is "Customer Collection" - because we have a physical store I want those local to us to be able to call in to collect orders.

When it comes to payment options - I have added one (by amending the "COD" module) for "Payment on Collection".

Clearly Shipping as "Customer Collection", and Payment as "Payment on Collection" is fine.

However as it stands someone could place an order and select Shipping as "Royal Mail 1st class", but then go on to choose Payment as "Payment on Collection" - this wouldn't be a valid combination.

I'm guessing that the Payment module for "Payment on Collection" is going to have to know what Shipping method has been selected? Is it posible to restrict it in this way as it stands, or does it need some code twiddling?

Thanks,
David Spink
Zen rules!