Question about credit cards/credit ratings

Hi ukpersonal finance. I have a question for you. I have a credit card that is on an 18 month interest free period. I use it to spread payments for larger purchases over a few months. I plan on clearing the card before the interest free period expires before applying for a different card with a fresh 0% on purchases offer.

I have two questions; firstly, how will this behaviour look on my credit score? I don't clear the balance in full every month (which I know is most favourable) but I pay what I can which is always more than the minimum payment.

Secondly, my plan was to close the original card before the 0% period expires. I have heard, however, that it can be good for your credit score to have access to credit that you do not use. is is it better to close the original card before applying for a new one or to just leave it with a zero balance?

Any advice would be much appreciated.