Well, I have read the 2002 statement now and I do not share your take on it, Holly Goodhead. I should mention I was practising criminal law at one of London's leading firms in the 80s and I maybe know a little more about these things than some. The purpose of that statement is to address the suspicion that Mugford gave her evidence in return for immunity from prosecution for the cheque fraud. There is absolutely nothing intrinsically improbable about it as far as it goes. Had I been acting for her and/or Sue I am pretty confident the bank could have been persuaded not to press charges given the freely-made admissions and the offer of recompense. Banks don't tend to see either every day and honesty, being thus rare, tends to be well received in these situations.
However, what is glaringly obvious to me is the much larger problem presented by JM's evidence. In telling her story the way she did, she admits the offence of 'concealing' (see CJA 1967) murder. That is extremely serious. Far more than any cheque fraud. She could have been imprisoned for many, many years for that offence, of which she was undoubtedly guilty. Also, unless you were born yesterday, you cannot seriously believe that, on her own account (which she need never have contrived if she were making the whole thing up) she spent the best part of a year listening to Bamber planning to murder his parents, helping him commit one crime against his own family that we know about and supplying him with temazapam for the declared purpose of letting him to test its efficacy for use in a murder plot without having been involved in some degree.
Now, the police may be stupid but they aren't that stupid and even if they are, prosecuting counsel certainly are not. The really big question is therefore not about how they got off the cheque book fraud but about the much more grave offences I have described. It's not surprising this aspect has not come into focus. The reason should be obvious. The only person who could bring it up is Bamber but he can't do so without blowing the whole charade.
If the cops had conclusive evidence of his guilt such that they did not need her, she would have been in the dock with him (on the unrealistic assumption that she had confessed in the terms of her statements).