But why hasn't Virgin bought this "serious" offer to the table earlier?

The reports do not indicate anything that I can see that would have prevented Virgin making this offer some time ago in order to "study the books". If the current indications are correct they have made a safe "stake" offer that allows them to seem to play but seems insufficient to yet prove to be a satisfactory deal-making bid. So far nothing has been spent, and nothing is at risk.

So, is it simply a "spoiler" ie at no cost at all it enhances Virgin's own repuation and possibly pushes the cost up for IAG? It also is to the benefit of LH as it assists LH by providing additiona leverage in it's discussions with IAG is it has been up to now has been a one-horse race?

There's nothing I can see that would have prevented Virgin tabling this offer earlier and it needed to see the books before now if it were serious about optimising the deal.

Too many unexplained oddities so it may be a serious bid, but at the moment I'd still not put money on it.