The defence being offered by MP's confronted by prima facie abuses of parliamentary expenses is that they acted "within the rules" and that these rules need to be changed.

I thought it might be interesting to have a look at these rules. I was quite surprised.

...The Additional Costs Allowance (cost of staying away from main home/ACA) is paid to reimburse Members for necessary costs incurred when staying overnight away from their main home for the purpose of performing parliamentary duties.
source:- www.parliament.uk/about_commons/hocallowances/hocallowances0...

...It is your responsibility to satisfy yourself when you submit a claim ...that any expenditure claimed from the allowances has been wholly, exclusively and necessarily incurred for the purpose of performing your Parliamentary duties...

...Only those additional costs wholly, exclusively and necessarily incurred to enable you to stay overnight away from your only or main UK residence, either in London or in the constituency....

...The Additional Costs Allowance (ACA) reimburses Members of Parliament for expenses wholly, exclusively and necessarily incurred when staying overnight away from their main UK residence (referred to below as their main home) for the purpose of performing Parliamentary duties. This excludes expenses that have been incurred for purely personal ...purposes.

...You must ensure that arrangements for your ACA claims are above reproach and that there can be no
grounds for a suggestion of misuse of public money. Members should bear in mind the need to obtain
value for money from accommodation, goods or services funded from the allowances...

...You must avoid any arrangement which may give rise to an accusation that you are, or someone close to
you is, obtaining an immediate benefit or subsidy from public funds...

...ACA must not be used to meet the costs of a mortgage or for leasing accommodation from:
 yourself;
 a close business associate or any organisation or
company in which you - or a partner or family
member - have an interest; or
 a partner or family member....

...The following expenditure is not allowable...
...The capital cost of repairs which go beyond making good dilapidations and enhance the property...
...Furnishings or fittings which are antique, luxury or premium grade

source:- www.parliament.uk/documents/upload/hofcpsap.pdf

I'd have to say that the above seems reasonably clear to me and it seems pretty clear that many of the abuses which are being revealed are in blatant contravention of the rules.

A multi-millionaire minister spending money to bring her preferred chimney sweep in from her constituency base to sweep the 8 chimneys in her Soho house? Wholly, exclusively and necessarily?

The same woman claiming for the repair of an antique Chinese rug? Wholly, exclusively and necessarily?

Hazel Blears staying in a London hotel classified as one of the 50 coolest hotels in the world whilst she was in between second homes - said second homes being run as a mini property development business so she could do them up at public expense before selling and moving on to her next project - three in one year? This is an expense Wholly, exclusively and necessarily incurred to do parliamentary duties?

The fault does not lie in the rules. the fault lies in those MP's - a large majority??? - who abused the rules like any other dishonest employee fiddling their expense accounts

and also in whoever was supposed to be policing/enforcing these rules. They have clearly colluded with flagrant abuse.