I have been watching D-50s on eBay for more than a year (on my short list of Guilds to buy). A '79 D-50 recently sold on eBay for $750... but this was an anomaly. Very spartan ad, with little info on the guitar. More typically, '70s era D-50s in decent (VG) shape sell from US $900-$1400 (often outpricing D-55s from the 80s and 90s). An early '70s D-50 burst "special" (with block fret markers like a D-55) sold for ~$2300 a year ago. Prices in shops and Gbase tend to be ~ US $300-500 higher. That one looks to be in fine condition... in the States, I would expect it to easily go for $1000-1200 (550-600 pounds). But, I hear Guilds fetch a premium price in the UK, so wouldn't be surprised if it went considerably higher than that (600+ pounds), but won't hazard a guess.
From the grain and silking, and from what I have read re: spruce woods, I would say all the tops of my 70s-90s dreads are sitka. I would guess this one is, also.