I agree wholeheartedly with Flyin'Dutch'.
Main question to ask - Why was the C of A not renewed?
There may be a number of reasons, from the obvious run out of money, love, interest (delete as applicable), but it is also highly possible that the engineer who was previously looking after it was willing to let some defect remain until the expiry of the C of A, whereupon they would need fixing before the C of A was renewed. Another strong possiblity is that he was not happy for the engine to go onto extension and run on condition for some reason (Compressions/ Oil consumption?).
Supplementary questions -
How long since the C of A ran out?
How old is the engine in calendar years?
If it has stood for a while, what corrosion prevention measures have been taken?
If you proceed with this, I suggest that you speak to the previous engineer and try and find answers to the above questions. If you are satisfied by those answers then I would get it surveyed by an independant engineer - No point spending money on a survey unnecessarily!
In terms of value, there are a couple of flying 880b's floating around for sale at the moment at around the £10/12k mark.
Until proven otherwise I would value this one at it's scrap value - possibly £2/3k - you could easily spend £10k+ to get it flying again if it needed an engine!
The Rallye is a fantasic and very under-rated aircraft - I own two Rallye 180's (893's), but thats another story................