EviLClouD: Just an update - got a few more quotes.
What are your thoughts on solution dyed polyester carpet?
Also with the underlay, is it worth going to rubber underlay? Or even thicker? The quote is 9.5mm 95kg underlay which i assume would be foam?
Solution dyed polyester has been discussed above. I would definitely go with the thickest underlay you can, it's insulation and padding. I'd look at rubber underlay next time, but I haven't ever tried it.
IMHO the (only?) drawback of the modern SDN carpets is that they look like SDN - the consistency of colour and the nature of the way light reflects off the strands means it that to me it is an inferior looking product compared to wool.
Once you've spend a few hours comparing carpet samples you'll know what I mean. There's a certain wool look that none of the synthetics can match.
In a modern home I'd go SDN in a heartbeat, but if you are redecorating an older house and want to keep your carpet sympathetic to an older style of decorating then wool is the best choice by far.