Hi DH!
I have a later one, 1984 Westy, think it was around £300, has had all the wood treated etc, and has different suspension, it seems to be built in to the swing arms rather than those on the early models.
The early's will probably have a slightly higher value maybe? Yours has the wooden top with was a extra, and is a nice addition.
Spares available here:
http://westytrailers.com/en" onclick="window.open(this.href);return false;
Dampers about £70 a set
I have seem them from around £250/300 to £475 for real nices on ebay, there was a dealer selling them on ebay a while ago, may have been Franks Westys Parts?
This one is £400 on ebay:
http://www.ebay.co.uk/itm/Westfalia-Suitcase-Trailer-1970s-Refurbished-And-New-Parts-/111243766172?pt=UK_Car_Accessories_Touring_Travel&hash=item19e6a5299c#ht_158wt_1275" onclick="window.open(this.href);return false;
So in answer to your question, £300? But as always, it's whatever a person is prepaired to pay.
Good luck with the sale DH
Alistair