Can anybody tell me if A series filters are suitable for use with the PD150 or should I be looking at the P series? If anybody has experience of using these filters with the PD150 I'd be very grateful for any advice.

I'vve used some A stuff beofre and they were fine. BTW, nice name, albeit, you spell it "wrong", ;P

Why don't you just buy some screw-in Cokin filters?

P series will give you a greater range of filters.
Singh Ray, Lee and Hitech all make resin filters to fit the Cokin system. The Cokin adapters and filter holder are kinda mickey mousebut you can buy an aluminum 2 or 3 filter rig from Hitech.

You could even go with the lee or Hitech 100 mm filters and filter holders. lee sell adapter mounts so that you can use a P filter in a 100mm (4"x4") mount

http://www.2filter.com/prices/products/hitech8.html

http://www.formatt.co.uk/hitech/85_page1.asp

Thanks for the information, guys. I want to experiment with multiple filters and graduated filters. I've got a few Tiffen screw-ins but don't want to stack too many in case of vignetting and want to have more control when positioning graduated filters. Unfortunately a matte box and 4x4s are way out of my budget at the moment. The Cokin system seems a much cheaper alternative. Thanks to your advice I'll give the P series a go. BTW the Hi-Tech range looks really interesting and a little less Mickey Mouse - thanks for the heads-up.

Plus the Hitech are made in the UK. (Lee also)

look at the TLS Kestrel and the Van Diemen Mosquito soft boxes as well. Formatt have one but in a high wind you may fly.

For price
http://movie-house.com

For info
http://www.truelens.co.uk/kestrel.htm

http://www.vandiemen-ltd.com/pl_main_matteboxes.html

Have a look at cavision's 3x3 system, it has a rubber shade and one fixed and one rotating stage. The plastic clamp is $200 US ( 120 quid) and the aluminum clamp mount is $250 (150 GBP). You'll need a $5 adapter ring if the OD of your lens is below 85mm. The filter trays are machined aluminum. I've seen this and it's a really good deal. This one's out of Canada, so you'll be keeping it in the family :D
http://www.cavision.com/Mattbox/3X3Rectanglar.htm