Now there's a question - how long is a piece of string
I guess you've got to work it backwards - the youngest desireable comp III's are the rainbows which you see an NOS set fairly regularly go for between 30 and 40 quid depending on thin/thin, thick/thick, thick/thin set-up. Then you get to blue/green label and you're gonna see a rise of 10 to 20 quid on top of that depending on the afore-mentioned size combo and obviously the condition and then i would say a similar price step again maybe to the yellow label's all things being equal.
Without a decent picture and honest description of condition it's hard to say but i would say 50 quid + just from your description without photos........
.
.
.
.
.
.
.
.
.
Post some pics