My question would be why there are "so many" MPs falling foul of the rules ( irrespective of political partisanship).
I refer my learned friend to my previous analogy of registering hospitality.
This is about declaring things so that we're aware of what might influence our MPs. In their case it goes beyond accepting gifts of biscuits or attending a lunch or dinner and includes earnings for speaking etc. It's got nothing to do with paying tax; freelance/self employed earnings have to be accounted for to HMRC just like anyone else.
Reporting this stuff timeously and accurately is a task for MP's admin. In an ideal world an MP has a designated staffer who's on top of this, picks up what he can from office records and, as the appropriate deadline approaches sits down with the MP and says 'anything for the Register this month?'.
In the real world of overwhelming constituency work, an MP who is politically brilliant but an organisational disaster area*, and the general blood and guts of the battlefield stuff falls between the slats.
*I worked for a while with a Professor who was constantly sought out for his expertise. Don't ask him to complete his own expenses form though. Book him a hotel and he'd not retain the detail; call from Heathrow asking to be 'reminded' where he was staying in London.