Most will agree that our MPs need a second home. They need a base in London within easy striking distance of Westminster. It's not enough to suggest that they check in at a hotel or a B&B. To do a job like that you need a home where you can just turn up. Most MPs do not use the second home as a pad to put their mistress up in. Non-London based MPs only really need a small flat.
It's also reasonable that certain expenses could be claimed for the second home. They're paid more money than I'll ever make but it's peanuts compared to some London salaries.
The problem arises when they start to juggle the numbers, declaring their "second home" as the one that will benefit them financially the most.
So, whoever is in charge of these things should simply declare that for purposes of Parliamentary expenses, the second home is a wholly-owned property (no more renting bedrooms from siblings) that of all the MP's properties is the one nearest Westminster. Peasy.