OK after spending all that time and deciding to post, I have now sorted it out. Have had to use the VLOOKUP and put all the four parameters in and also put a False at the end of it and it seems to play quite nicely now. Still can't figure why using the simpler 2 parameter LOOKUP did not work.
Have to try a little longer next time before I post - was at tearing hair out stage, if I had any to pull out!!
Will close this now I have sorted it