Hi Gareth, welcome to the forum,
First the bad news;
A 210mm 24 tooth blade is for rip sawing and not cross cutting and should not IMHO be used in a mitre saw.
Why do they put one in? Because they are cheap
I've had a look and can't find a blade the same size as the one you have.
Now the good news;
As it is in a mitre saw you could use a blade which is slightly smaller than supplied and Screwfix sell a 205mm 60 tooth blade with an 18mm bore for £21.99. Some Screwfix own brand is v poor so I can't recommend it but it is an option.
I would go for a saw blade reducing bush from Axminster power tools part no SBB018 for £1.88
http://www.axminster.co.uk/default.asp?part=SBB018This will allow you to buy any blade sized between 200mm and 210mm with a 30mm bore.
For instance,
Perform 200mm 48tooth £11.52 Axminster
Freud 210mm 40tooth £26.95 Rutlands