Thanks for the reply - you're probably right. I just wonder if Paypal have extra fraud prevention measures in place which exacerbates the issue. To give you a feel we see around 20% of our cancellations per day are as a direct result of declined transactions (for this very reason "card declined by issuer").
It just "feels" too high and one that we'd have assumed would affect hundreds/thousands of other SaaS UK companies who bill the bulk of their customers from the US. But there are few (if any) reports of it online.