When I chose 'invisible' in preferences briefly and acknowledged that my friends would disappear I accepted this. What I didn't realise was that after coming back that I could not send friend requests to people I have previously sent them to even if they accepted the first time.

This means I have to send a message to each person asking them to friend me.

Is this intentional?

Yeah, it's intentional. Once you go invisible we have to wipe your friends list, but it doesn't wipe record of you having requested them before, and you won't be able to request them again. It has to be this way because if it wasn't, people could reset their own friend request history just by going invisible then visible again.

Ah ok I won't be doing that again then!! In future if someone winds me up I'll count to 10 before clicking the 'delete'/'invisible' button

No worries. I actually need to make it more clear that you won't be able to re-request those friends. I'll do that soon