Marton Mills are very helpful,and really will try and supply you fabric if they can.
They will weave what they call "Karl mayer"sample lengths of about 13 ms of many of their fabrics and when they are asked for specials, but my experience is that the costs are very high for the small lengths, infact on their latest price list they charge the same for PV as they do for wool .
An example of their PV (Balmoral)would be £35/m double width against £5.65/m double width on their standard range.
This is not to say don't do it, sometimes if you want a special fabric it is worth spending the extra.