Neutral Density Filter question

I've just bought a B&W ND filter ND 1.8 64X.
I was expecting to be able to do some very long exposures (30 seconds or so) with it, but it only offers about 2 to 4 seconds to achieve correct exposure.

Any idea which filter I need, or should I get another one, ie. can they be combined and used together?

They can be combined. Just watch out for vignetting. You can also use a CPL to gain a extra two stops. Just set camera to lowest ISO, stop down to F16 and you should be able to get into the long exposure area.
Obviously it al depends of the light levels, you are dealing with.

I have a set of ND2, ND4 and ND8 filters.(1,2 and 3 stops.)

Martin.

If the situation/subject permits it, you can use the in-camera multiexposure.
I've used it in my K10D and it works quite well for say... a stream or waterfall.
Nonetheless an ND is a great tool to have.

It seems short though the times you're getting can you give us more info on what kind of settings your camera is on?
If you finally come in the seconds, times are getting bigger faster so squezing your aperture 1 or 2 stops might already give you times of 8 seconds, I believe.

If you're going to stack ND filters you need to multiply them,
so ND4 and a ND8 would be a ND32.
The ND64 is a very heavy one so I would go for a ND4 or ND8 next to it.

I've just taken a few shots outdoors at f22 iso 200 (this is min iso on k100d) and correct exposure time is only 3 secs.

I want to achieve shots lasting much longer than this, say 1 or 2 minutes.

Am I correct in thinking that to combine an ND8 with the 64x filter, I would then get 8 x 3 seconds?

Yes I believe so Karl but don't take my word for it.

Buy a Bigma and the light will automatically dissapear for you