After contacting the guys on the Jeelib Cafe forum I was made aware that the CRC errors was a library bug and this had been fixed in a fork to the original Jeelib library (which is now about 2 years old). So, after installing the forked library I was indeed able to confirm that the issues had been solved.
I left two units, one transmitting and one receiving, running overnight sending gibberish test data and this morning it was still running without one single lost packet of data.
A lot of time was wasted with that. Anyway, now it is fixed I am able to get down to the nitty gritty of connecting up a sensor and transmitting real live data over the network.