I have just had a Talktalk Wi-Fi extender kit DHP-W611AV installed which so far works well. However this kit is a replacement for a D-Link network connector kit. Is this redundant D-link network kit in any way compatible with my new Wi_Fi extender kit?
Depends what standards your older Network Extender Kit supports. I assume you're talking about the powerline standard?
The Wi-Fi extender supports the AV2 Home Networking standard and this is compatible by design with the earlier AV Home Networking standard. The downside as usual is that connected devices work at the speed of the slowest device. So you'll not get the full quoted performance out of the faster AV2 product.