Hi Jo
The idea you are trying to implement is actually moderately simple. If I give you one example I am sure you will be able to apply it wholesale.
Field 1 and Field 2 are my only fields
I want to validate the entry on Field 1 but not stop people from submitting the form if left blank
My validation will read something like: Where @Field 1 is not blank AND where @Field 1 does not match pattern "custom pattern" \d\d\d\d\d\d\d represents a seven-digit number. This way if @Field 1 is blank no validation applies, if however it is completed it has to have 7 digits in it.
FYI the custom pattern screen details all manner of different characters to build your own special pattern for the field.
I hope this helps