There's something very odd about this.
In England & Wales the last date for the very last area to be subject to compulsory 'Registration of land on 1st transfer' was reached in 1990. Many areas of Britain had been subject to this since way before 1990. This means that, since 1990, no land whatsoever (in England & Wales) can be transferred (sold) without using the Land Registry to record that transfer. You say you bought the land in 1997, 17 years ago. So that sale HAD to be registered, so something is wrong here.
The purpose of a 'Caution on First Registration' being recorded at the LR is to alert an interested 3rd party to any attempt to sell a parcel of UNREGISTERED land. There's still plenty of unregistered land around - it just hasn't been sold for donkey's years. But yours should not be one such plot (see above).
I smell a rat here - I suspect there was some kind of dispute as to where the boundary should go and the owner of the other plot (who sold you your plot) registered the caution with LR to prevent you doing anything to sell it until you sorted the issue out. Perhaps he was effectively holding you to ransom until you sorted it out.
Whether it was this that prevented you from getting the land registered at LR 17 years ago is anyone's guess - you should not have been permitted to carry on like this for 17 years (i.e. make the land purchase without registering the land; I assume that you merely have the original transfer deed). Did a solicitor deal with your purchase?
It certainly seems to have caught up with you now, and I am sure that you can see that any prospective purchaser is going to insist that you resolve it before agreeing to buy. Unfortunately it is your issue to sort, not the new buyer.