Many thanks. I was slowly coming to that conclusion myself.

I have now added a duplicate parameter (@param2) that does not use available values but is part of the where clause with an OR

So I have changed SQL where clause from:-

where customer in (@param1)

To:
where customer in (@param1)
or customer in (@param2)

This works as required. Just a faf! I can now just supply param2 in URL with out param1 and I get the data I want 😀

Thanks

Kelvin