I've used senditnow to post wheels/frames without any problems in the past, but I need to send 4 or 5 bike box sized parcles (not bike stuff though). Any advice on who'd be the cheapest to use?

It really does depend – they all seem to have different pricing models that they work from, with one company better for small fat parcels & another better for heavy stuff etc.
It's probably worth getting an online quote from a few & see which one comes out best for you.

as far as service goes – they're all about the same
all the ones like senditnow, parcel2go & interparcel are just agents & use whoever they can get the best deal from Parcelforce, DHL etc.