Right at the heart of the Scottish Youth Theatre’s production of Two Weeks with the Queen is Sean Biggerstaff’s diamond-hard performance of the boy in search of the miracle cure for his cancer-stricken young brother. Bright with the optimism and “can do” of youth, and with the cutting edge of uncomplicated honesty, he carves a path through adult evasion and hypocrisy from Sydney to London and back again.

It is really an extraordinary piece of acting from this third-year Glasgow schoolboy, already with an Alan Rickman film behind him (The Winter Guest). Playing a younger self is always testing, but he gives full expression to this cricket-mad Australian schoolboy, wide-eyed with excitement at the people and places of this planet, but with a determination driven by the “He’s not heavy, he’s my brother” philosophy.

Just occasionally, in his need to keep pace with the tempo of the character, he loses articulacy in his fluent “Strine”, but this is more than forgivable in a jewel of a performance, all the more remarkable for its eccentric setting. We see this fair dinkum, all-Australian boy, except for a brief prologue and epilogue, only in a strip-cartoon London.

The caricature serves well for some Aussie pommie-bashing and anti-monarchy propaganda, but leaves the rest of the young actors with some cardboard cut-outs to walk round the stage. The general air of unreality infects the plot: our young hero’s attempt to break into Buckingham Palace is followed by a plan to interrogate a tribe in the upper reaches of the Amazon.

Director Mary McCluskey catches this mood exactly when she stages a skilful tribute to Dennis Potter, the cancer ward nurses and doctors miming their way through Bing Crosby’s “Accentuate the Positive”, but it is in the hospital waiting room that the script finds its feet again.

There the boy hero meets two homosexual lovers, one visiting his Aids-stricken friend, and befriends them in his cheerful, uncomplicated way. Now the writing changes key, the characterisations become real, the authorial stance sympathetic. Particularly, the homosexual partners are Welsh, and worked together in a steel foundry.

We are quickly into Aids Awareness territory (our young hero even shares an orange with the terminally ill patient), and importantly, the boy is made to recognise that his brother’s illness is incurable, and that all he can do is love him and be with him to the end. It makes for a strong ending in what is a difficult but technically very accomplished production.