I've used Interparcel and ParcelMonkey plenty times without issue.
For oversized things (such as bumpers) I've been using Courier Queen on eBay - they are effectively a booking agent that use City Link but feel I have some sort of comeback on failed collections as they advertise 'Bumper collection' etc. Costs about £14 for a bumper next day, used them about three times this year - never had an issue.