[mothered]

I should've taken advantage of it when it was instock

Unless It was on back order at the time, you couldn't have known further down the track that they'd be out of stock.

I would've done the exact same.

I could go for the 6TB but it saying it weighs so much less is a put off. And I'm not comfortable risking loosing £160

It's not worth the risk.

The method you're using, Is heavily based on the precision/approximation of the Item's weight.

It did was it was low in stock then they kept saying they were in stock.

Still, It's difficult to know exactly when they'd run out of stock, If ever.

As It stands at the moment, you're waiting on stock replenishment/availability.