My 10-year old Raleigh Trail 4000 commuter bike failed me for the first time last night when the 8-speed Sachs twist grip started slipping. Had to borrow Mrs Salmon's mountain bike this morning.

When I took it off to have a look the spring that indexes the changes had snapped and I think it's probably terminal.

What are my options for replacement? Will any 8-speed twist grip work e.g. SRAM? The rear mech is a Shimano so is that important?

So far I've found this: SRAM ESP 7.0 Twist Shift Shorty 8sp + Grips that looks appropriate.

Alternatively, can I replace with a trigger shifter?

The relationship between a shifter and a Shimano rear derailleur is that there is a 2:1 ratio of cage movement to cable pulled.  Or is it the other way round?  Not sure.  Doesn't matter; the point is the 2:1 ratio.  SRAM do both 2:1 and 1:1 shifters, but the 1:1 shifters are for their own kit and won't work for you.  The ESP shifters, so far as I can tell, are 1:1.

When my twist shifters died on my mountain bike I changed them for SRAM Attack trigger shifters, and they have worked very well.  Another bike uses Shimano Alivio trigger shifters, and they also work well.

Thanks Colin; I would never have known all that.

Are these the ones http://www.chainreactioncycles.com/Models.aspx?ModelID=13206?

Does it depend on what type of Shimano derailleur I've got?

That's the ones.

Doesn't matter, in general, what type of derailleur it is, as long as it's the same ratio.  Since it's Shimano then you'll be fine.

An update if anyone's (still) interested.

The SRAM MRX Comp was extremely easy to fit and works perfectly with my Shimano set-up. In fact the gears are shifting far better than they have in years.

A big-up also to Julie's Cycles in Leicester who delivered the shifter in just two days.