They did have a STPC (Stopover package) at Jakarta for those with long stopovers - but I can't find details, so maybe they have removed (or I just can't find) - it used FM7 hotel. So i'd call back and ask what hotel they will be putting you in. Use the 'hang up and try again' approach.

If you get no joy then I would suggest -

1- do check and see if there are better connecting flights with other airlines at a similar price (probably not!) for you to book if you were to get a refund (refund would probably be slow though so you'd need to shell out funds upfront)

2 - do look at FM7 - normally around £30 and a good place to laze around the pool and bar and eat until your onward flight.

3 - or turn it into a mini break - request rebooking onto the following flight and spend 24 hours in Bali instead:

09 DEC GA 87 LHRCGK 2155 #1855
10 DEC GA 412 CGKDPS 2050 2345

11 DEC GA 718 DPSMEL 2130 #0600

Gets you into Melbourne 75 minutes before the Jakarta - Melbourne flight! Be surprised if they deny you this as it's a valid routing from LHR-MEL.