If the Carillion scandal was caused by back handers from Carillion boss to the Conservative Party is this not corruption?
It's not exactly 'backhanders' because donations to political parties are entirely legitimate; it's the only way they have, apart from membership fees, of raising money to finance their operations. But it does look very like corruption when companies and individuals are paying very large amounts of money (£25,000 in some cases) to belong to 'donor clubs' which guarantee them direct access to members of the government. And it is equally so if large donations to other parties, such as union donations to Labour, give them preferential access.
As I said earlier in this thread, I think that state funded parties might well be the way to go, with no additional donations, apart from realistic (i.e not highly inflated) membership fees, allowed. It would create a more level playing field and promote a more democratic access to 'power'.