If one surveys the Scottish theatrical landscape, one of the most immediately apparent features is the increasing prominence of A Play, A Pie And A Pint (PPP). The lunchtime play series, established in 2004 at the Oran Mor venue in Glasgow by the late, great producer David MacLennan, now ranges elsewhere in Scotland and, on occasion, further afield.

The latest play, Val McDermid's Margaret Saves Scotland, is a case in point. Having premiered at the Oran Mor, it tours to the Traverse Theatre, Edinburgh in the coming week and the Lemon Tree arts centre in Aberdeen the week after.

All good, one might have thought. Except that PPP seems to have fallen victim to the law of unintended consequences.

Staging one new, short play per week throughout much of the year, MacLennan's brainchild was inevitably going to become Scottish theatre's most prolific producer. However, PPP, which creates predominantly light-hearted mini-dramas on shoestring budgets, was, surely, intended to complement existing new theatre-making in Scotland, not to replace it.

However, with the programmes of the Traverse (which declares itself "Scotland's new writing theatre") being pared back and Scottish theatre's touring circuit having shrunk noticeably, PPP appears to be playing an increasingly central role. As McDermid's little three-hander attests, this is not a role for which the lunchtime series was designed.

One of Scotland's most famous and successful novelists, McDermid makes a rare, and, one has to say, not entirely successful, foray into theatre with a far-fetched and politically romantic tale.

It's 1958, and nine-year-old Yorkshire lass Margaret Holt, inspired by a family holiday to Kirkcudbrightshire (where her mother was stationed during the war), has, unlikely as it might seem, become an ardent Scottish Nationalist. It's not long before she's stowed away on a potato truck to Angus, determined to lead an uprising that will re-establish Scotland's parliament.

Margaret's childishly idealised notions of Scottish history can be excused by her youth. The fact that the play shows every sign of agreeing with them is more problematic.

The Treaty of Union is represented as an act of English domination, rather than the consequence of Scotland having been bankrupted by the "parcel of rogues" that was its ruling class at the turn of the 17th and 18th centuries. Likewise, Jacobitism is portrayed as an uncomplicated expression of Scottish national identity, not an ill-fated, rearguard action against industrial and religious revolution.

This rose-tinted historical narrative is rolled up in a comic-musical bundle. The cast of Tori Burgess (Margaret), Simon Donaldson (Tam the tatty seller, among others) and Clare Waugh (Tam's wife Jean, and others) do a fine job of acting and singing their way through a script that is part Ealing Comedy, part White Heather Club songbook.

Old-fashioned and inoffensive in theatrical terms, and just plain silly in terms of political history, Margaret Saves Scotland is a straightforwardly disposable wee play. Lunchtime theatre can afford to be such at times. What it cannot be is responsible for the fortunes of the country's new theatre writing.