Ok, so I feel a little stupid now.
I altered my network around on Monday but did not think to update the network settings on the Humax.
The gateway adress and dns adress were no longer pointing at my router, but to another router that was no longer the Internet gateway.
Now, as long as you start the humax with the network cable unpluged and leave it that way, the box will not crash, even when viewing the affected channels. If you start the box and then remove the cable it will crash only when trying to view one of the afore mentioned 4type channels.
Now that I have corrected the dns and gateway my system seems stable again.
However, I have had this crashing problem before, and it usually seemed to coinsided with a powercut earlier in the day (we get alot in devon). On these occasions I could verify a network error by simply not being able to ping anything the other side of the network switch or open humax url in browser (webif). That was a sure sign that the home plug had failed to renegotiate its connection after the powercut (we have a few of them about the house and they have to start up in a timely order it seems for them all to play nicely) . Simply switching the homeplug of and on again would cure the crashes.
I hope this at least helps focus the problem, as im sure a problem does exist, just that I was forcing the problem to be more prevalent by having a conveluted route to dns etc.
I would sumerise that if the humax is powered up with a network cable attached, and there is a subsequent network access issue, it forces a crash event to occur when viewing certain channels.