The useful replies above (thanks both) got me thinking, about whether I'd be happy with an 'open' bivi. which led me to finding...
Dutch army issue GoreTex hooped single hooped bivi hich looked ideal
but they are rather expensive compared to a Hunka (£90 upwards vs £40) but then I found one on ebay from a surplus store in grade1/ supergrade condition with no bar or pole for £50.
Bought that thinking I'd use a spare tent pole but then found another supplier who does the original curved alloy poles for £20.
The bag itself is fab quality & hopefully the pole will arrive tomorrow. Pics to follow. yay!