I purchased the new PRT Thermostat kit from Elise Parts last month as I was very worried about the temperature control of the K series, I was seeing a warm engine shocking down to 58deg. I know that the PRT was the saviour of the HGF on Freelander and so decided the conversion had to be done.
The Elise Parts kit is much more expensive than the Land Rover Kit than can be converted, but it does have all the parts needed and Silicon Hoses.
Fitting time was indicated at about 3 hours, mine took closer to 5 hours. The kit is very easy to fit (there was no silicon grease included to fit the old thermostat casing back but Elise Parts tell me they will now include some with the kit, need to make sure you have this as finding it locally was not easy.)
With the rear of the car on ramps the undertray came off easily giving good access to drain the cooling system. Removing hoses was easy and even removing the water rail with engine not removed was not too bad, it just takes a while to get the right approach angle and use wobble drives. I found removing the air box also made life much easier getting to the water rail bolts.
The silicon pipes fitted well and there was only one foul with an alloy pipe going down the side pod that needed some clipping.
As the car was raised at the back it needed only about 3 ltrs of coolant to refill and I used the Elise Parts additive to ensure that there is no corrosion (trust me, antifreeze is just as important as oil to an engine). Once filled, I bled the water rail and ran the Exige up with no leaks.
With the car returned to level the front rad bleed screw had no air in at all and the water rail had only a few tiny bubbles.
So was this worth the effort??? 
Very much so 
. The temperature now only varies by 3 deg at the most, there is now a feeling of confidence that the block/head is not constantly expanding/contracting. Compared to the cost of a HGF this has to be a worth while move from standard (I like my cars standard and apart from this and HID Lights the Exige is 100% orriginal).
If any one is interested in some photos of the conversion I can post them here. Also SELOC has some good articles on PRT conversions using the Land Rover thermostat.