It looks like a Vasart to me. I have a few and they seem to come in many diameter sizes.
If Monart (unlikely) the shape is probably Y ref Ysart site.
I think with that domed centre and use of aventurine it is a B029 Vasart, I have one posted on the Ysart forum. http://www.glassmessages.com/index.php/topic,3046.0.html
Later made Vasart I think was less likely to be acid etched to base, unfortunately this can have people thinking they have a Monart.
But hey, I am not the expert here. Frank is so post in the Ysart forum and he will give you a definite answer. I think the fact it went for £29.00 tells me it is more likely Vasart than Monart - too many eagle eyes out there LOL