The future, says Iain Stevenson of BAE Systems, is “fantastic”. His twin Clyde shipyards, Govan and Scotstoun, have orders for two decades and £100m in investment coming up over the next five years.

The defence giant’s Glasgow-based and Glasgow-trained engineers and designers are at the cutting edge of British and world warship-building. Their next big project, the Type 26 frigate is what Mr Stevenson, the concern’s navy division’s managing director, calls a “high-end war-fighting ship”.

The versatile anti-submarine frigate - just a few feet longer and it would be reclassified as a destroyer - “goes to war and wins”, he explained.

It comes at a high price too: the first three will cost £3.7bn. Another five are expected to follow.

And this is where the tricky politics comes in. Back before the heady days of the Scottish inde-pendence referendum, the talk was not of eight new frigates, but of 13. Eight would be submarine-killers, five more general purpose. But all would be high-end.

So excited was BAE Systems about this work that Mr Stevenson’s predecessor talked of a “frigate factory” at Scotstoun churning out the new machines. There was nowhere else in Britain capable of such manufacturing, it was stressed.

Three years after the referendum and further doses of state austerity and the number of Type 26s has dropped to eight and the frigate factory has been ditched.

“It would have been wonderful to build a brand-new frigate factory,” Mr Stevenson said. “Unfortunately the economics did not work out. The costs went up. It was a great dream to have but we have the best option now.”

This best option entails substantial changes across both yards - currently making five patrol vessels - and still relatively high investments. But it leaves questions about whether the yards have the capacity to do any substantial new work, including the cheaper ships, Type 31s, the navy now wants to take its frigate fleet to 13.

There is no guarantee these ships - potentially, says Mr Stevenson, wanted on a timescale “never done before” - will be made on the Clyde. Or even, at the prices and terms being mooted, that the Clyde will want to make them. The Conservative government is looking for competition under a UK-wide shipbuilding strategy out soon which expected to try to spread the warship industry wider than just one river, for the first time in decades.

BAE Systems reckons it could do at least some of the work - and is particularly eager to provide weapons systems and engineering and design. But it would have to get a new “drumbeat” for Type 26s - the rate at which they are made - to produce the actual ships. And it may end up assembling blocks made elsewhere.

That, said SNP MP Chris Stephens, “will only increase costs”. He and Labour colleague Paul Sweeney want hard investment on the Clyde. The alternative is that Type 31s may not be doable at all. Could nobody build these ships? “That is a good question,” said Mr Stevenson.