Is it worth buying one of these when checking out a second-hand car?   Also any recommendations as some say 'free' or '£3.99' and then you can spend over £20 on one?

YES.... if there is finance on the car it is owned by the finance company and not the seller. The RAC do one for a fiver so worth the money for peace of mind.

I couldn't find the link for the RAC one for a fiver, do you have it, please?

Yes.  Some years ago my son found a car he liked and put a deposit on it (it was being sold privately), my husband did an hpi check and it came up   that the car was an insurance write off.  We managed to get the money back on producing the printout.  Would always do one now it is money well spent.

I've never bothered, but would hate to say that to you in case there were problems, I think it's quite unlikely though :-)

The RAC one seems to be£15.50?

I found one for £4.95 and it came back that the car was a Category C insurance write-off.  Ok not the worse write-off going but a friend's partner warned me that in the area the car is being sold in,its pretty common place and there's some shonky garages who will do it.