Andy Burns was thinking very hard :
Though openWRT can cope with multiple non-authoritative DHCP servers, I've
not checked what it does, but probably gratuitous arp on behalf of the
address it proposes to lease?

It's a while since I last messed around with my routers and stupidly,
at first I tried setting the static addresses on my none OpenWRT
router, which is not the DHCP server, yet serve most of the IP's. I was
then puzzled as to why the IP's of the items on my LAN remained as
before, until it dawned on me.