I have a sample which I want to play at the same tempo as my track (it's slightly slower than the track bpm), are there any samplers which allow you to conform the sample to the track's bpm?

You can do it within Sonar, use the CTRL and SHIFT key and drag. The Audio may sound a bit off, until you render the clip, and you choose the algorithm. (They just improved the pre-rendered process though, but I haven' tried that)
https://www.cakewalk.com/Documentation?product=SONAR&language=3&help=AudioSnap.19.html
https://www.cakewalk.com/Documentation?product=SONAR&language=3&help=AudioSnap.23.html
 

Any sampler that allows for forcing samples to play back at project tempo should do.
 As Kamakazi mentions about Audio Snap is good for moving down beat transient markers around, but be careful, there's a learning curve and until you get used to it you could really make a mess of things.
Rapture Pro, Dimension Pro are great for sound shaping and sound design.
 

Or just hit CTL-L and see what happens. If that is a loop, that should be all you need to do. Bounce it to clip if you want better quality. Maybe you need to fine tune it in the Loop Construction View if the number of beats or original tempo are not detected right.

Audio warp isn't working on that particular wav file for some reason. Tried rapture but doesn't seem to have audio warp capability. Can anyone recommend a simple, intuitive vst that allows audio warping?

Again, Groove Clip looping (CTL-L) should do, you should not need loading it in a sampler. Kontakt does it, by the way. Anyhow, try bouncing it to clip first, then groove clip it. If you do not get expected results, open it in the Loop Construction view and check if transients, original tempo and number of beats were correctly detected. If not, correct them manually. BTW, I am assuming that your sample is rythmical material with transients that you need to follow project tempo. If it is not, just time stretch it.

Hmm, I tried groove clip looping and it matched the clip to the tempo but now realise I need it to be double the time of the tempo.

Master Exploder
Hmm, I tried groove clip looping and it matched the clip to the tempo but now realise I need it to be double the time of the tempo.

Open the loop construction view and halve the number of beats in the beats field.
 
Also, remember that any warping needs to be done to a WAV file. Bounce a clip to itself first if it's a REX or Groove Clip or has a region effect.