A couple of points for info, WHBM.
Firstly Heathrow Express predates the current franchising system. I recall it has a track access agreement (for 4 train paths per hour, each way) for a period of 25 years.
Secondly, HEX has never earned anything like an adequate return on investment. But it was never expected to, based on fare revenues alone. Instead, the investment in HEX is included in Heathrow's regulatory asset base (RAB), such that any shortfall against the target regulated return is effectively made up by the airlines through airport charges.
Thirdly, "the Spaniards" only own around 25% of Heathrow now, having sold down to a range of sovereign wealth funds, pension funds etc. Not really relevant, but just for info!