Hi
I live in the area but not sure if I'm about at the time (moving jobs to a local College and all that)
Get an OS map as loads of trails over the Preseli hills (mountains is pushing it a bit, although my wife insists they are; she's local from birth...)
If you're looking at the hills, start and finish at Rosebush, as a great pub and tea shop there (seperate business') and big car park. They have also just opened a walking/cycling/horse riding area in what was (and will be again) forsetry area close to the car park; follow the unmade road through the old quarry.
Careful riding the coast path as it's walkers only...
There is a growing website:
http://www.thewayoutwest.co.uk/category/mountain_biking that details some good riding in the county by locals.
Although Pembs is mainly frequented by roadies, mountain biking is VERY slowly growing.
Give me a shout when you're here and I'll see if me, my PACE and my mate, on his 'modern' are able to meet up; even if only for an evening ride/natter/beer. PM me and I'll send you my mobile number
Pete

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Current rides: Yeti AS-R 2004
For Sale on eBay: CBR Gatecrasher
Building to ride: Yeti 575 2005
Building to sell on eBay: Graham Tomlinson 531c Road Bike, Peugeot Fixie & X-Rated Snare BMX
Flickr:
https://www.flickr.com/photos/58541481@N07/