Shower head non-standard size?

[thenewb]

Hi all, my shower head is leaking out the sides. It's a square vertical one and I purchased what I thought would be a simple replacement (this one). However, the threads are different sizes! Some searching reveals that apparently all UK shower head threads should be 1/2 inch, which I believe is what the new one is. However, the old one is bigger.

Some pics of the old shower/head

Pic 1
Pic 2
Pic 3

What are my options here?

Thanks in advance

 

[thenewb]

I have this sorted now, in fact I just removed the part you see in one of the pics, I had assumed this was not part of the old shower head.