Market rumor and speculation that Dublin-based, U.K.-listed pharma company Shire is a takeover target come around with slightly more regularity than Christmas.

With increased focus on the tax advantages for U.S. pharmaceutical companies on moving to the U.K. in the light of Pfizer's attempted $106 billion takeout of AstraZeneca, it's unsurprising Shire is back in the spotlight. Its share price has risen by 22 percent since news that Pfizer was examining a bid for its larger U.K. counterpart emerged – without any evidence of a firm offer (although it may also have been boosted by the early release for a new eye treatment.