Luton SID's
 

I was under the impression the departures from 08 turned left/right before Stevenage. However there seemed to be quite a few departures yesterday morning (Sunday 21st) that all but continued on the runway heading (especially the Wizz flights). Is it simply the case that traffic was such that they could be routed that way or has there been a change to the SID?

You are correct, none of the Luton 08 SIDs involve maintaining runway heading beyond the LUT NDB.

Of course ATC have the discretion to send aircraft off-track to avoid weather - could that have been an issue yesterday morning ?

You'll see departures on Clacton (CLN) SID's going straight ahead fairly frequently when it's quiet. For a lot of the aircraft flying CLN SID's e.g. the Wizzairs, the point at which they exit the London FIR is pretty much a straight track from departing runway 08.
It normally involves London Control (who talk to the CLN's off the deck) co-ordinating with Essex Radar (Stansted) as it's likely the aircraft will clip through their airspace.
This is likely to be the case for what you saw yesterday morning as I'm guessing it was pretty quiet. However as the previous poster said, weather could have played a part also.
There are noise restrictions tied into the departure procedures but I wont go ito them. If you're really interested read the AIP...

Obviously I'm not an expert but it was fairly foggy Sunday morning. Thanks to both for your replies. All the best.