The laptop is now out of warranty, so I contacted Dell to see about a replacement battery. "Sorry, we don't support laptops more than 4 years old" was the response. The support person suggested I source a replacement battery on Ebay and find a local laptop repair place to do the swap.
I deliberately chose to purchase a high spec (for the time), expensive laptop so it would have good longevity and previously I have had good support from Dell. Seriously disappointed by Dell's response this time.
Will be giving careful consideration to repair-ability and long term support before making another laptop purchase.