You get a cheque for �500 net.
Depending on your personal circumstances, your gross amount will be higher than that. The difference between the two are usually just National Insurance and Tax.
Your employer deducts that from YOU, and pays the taxman on your behalf.
He then also has to pay 12.8% of your salary to the tax man as HIS contribution to your National Insurance.
The 12.8% will come out of the profit he has made in the business. Any profit he has made, he will pay tax on but this is based on the figure he comes to AFTER he has paid you and all the other expenses.
So, to answer the question, (I think). The employer does not pay tax on your salary apart from the National Insurance. But, as previous posters have explained, this does not mean he hasn't got any extra costs of employing you.
Does that make sense?