it still slips under pressure .. my cassette is approx 3 year old and chain only a week
Unfortunately I can’t afford expensive cassettes .. on a budget ..
If you want your chain to stop slipping, you must procure and fit a new freewheel. A chain and sprockets wear together. The chain elongates because of wear and after a thousand miles (say) the sprockets start wearing to the point that, when you fit a new chain, the sprockets are too worn and the chain slips on it.
Did the old chain slip? No? Have you still got it? If so refit and ride on. Otherwise, fit a new freewheel. From the image I agree with
@Lovacott 's guess that it's a freewheel (and my family has several bikes with freewheels too), but it could be a cassette. Neither are expensive but only one will fit your wheel. See
this Sheldon Brown page to differentiate. Image from that site: