If your iPad and iPhone have different subnet masks, that certainly is relevant. Both devices (indeed, all devices on your LAN) need to have the same subnet mask. Their IP addresses must be different from each other, but part of the same network range. For example, a typical setup with a plain-jane consumer router would have a subnet mask of 255.255.255.0. This is what's known as a /24 network, and is the most common configuration for home networks. The router's IP address would be at 192.168.1.1 (for example). Your various devices (Mac, PCs, iPad, iPhone, etc) would have unique addresses at 192.168.1.20, 192.168.1.21, 192.168.1.22, etc. They don't have to be sequential, but they do have to be part of the 192.168.1.x range.
Some routers use 192.168.1.x, others use 192.168.255.x, 10.10.1.x, or various others ranges as defined by the internet addressing standards.
It's not clear from your post how you set up the addresses for your iDevices using their MAC addresses, but they will need to have IP addresses in the same range as the router itself, or they will not be able to connect to the internet through the router.
Thanks D.R.C. I didn't give the iPhone a MAC address but did renew the lease after reviewing a similar thread.
I managed to get it working by switching the router off for about 15 minutes (I had done this but only for a short period of time before) and hey presto it has not (yet) dropped off on my iPhone.
Thanks for the info re the subnet mask, will keep an eye out for that.