You have to remember two things about Jeffrey Archer's A Prison Diary before you can even begin to watch this stage adaptation by Mike Darnell.

The first is that people in prison do not always tell the truth. The second is that Lord Archer is a convicted liar.

Given that, you can now sit back and watch Darnell's one-man show based on the first volume of Lord Archer's book, which concerns his time in the top-security Belmarsh Prison in south-west London where he served three weeks of his four-year jail term.

Darnell, a scraggier and older version of the peer, plays all the parts. He is Archer himself, he is the female assistant governor, the prison officers and the various inmates with whom Archer becomes involved.

Thus we hear the pathetic stories of the inmates, most, apparently, in for murder and other serious offences, although there are some sad cases such as one man in for driving without consent and another of a l7-year-old supposedly on remand for shoplifting.

And Darnell/Archer tells one unbelievably sad story of a man in his 40s, sexually abused in children's homes and virtually robbed of a life who is in for murdering one of his abusers, a crime he says he did not commit.

The problem with all these stories is you never know whether or not they are true. Neither Darnell/Archer nor the characters as envisaged are properly rounded and even in Archer's own case, the reason for his imprisonment is not mentioned.

We get a look at the absurd rituals of prison life, the constant asking of name, age, height and weight, descriptions of the cells, the bad food, the interminable body searches and the seemingly pointless walking around and around the exercise yard.

There seems to be a great many petty rules and absurdity where it is better to take heroin rather than marijuana - heroin can be flushed out of the body within 24 hours while the other can take three weeks to leave the system, making it easier to beat the mandatory drug testing.

But this prisoner's diary is not a great prison memoir and written by anyone else, would probably not have been published. The only message is that there must be reform of the prison system - something which is far from new.

Darnell makes a pretty good job of the adaptation but the result is really only mildly amusing and only mildly entertaining. Thankfully, the less than 30-strong audience on opening night was more enthusiastic than myself.