Problem with Apollo Gradient clunky and missed gear changes

The first thing to check is that the rear derailleur indexing adjustment is good. I.e. the derailleur cage is aligned properly with the individual sprockets in each gear. If cage is moving slightly too far at each shift it could push the chain two sprockets over instead of one. I would expect a repair shop to be able to adjust indexing properly, and bad indexing causes the chain to rattle on the sprockets - which you don't mention - so this may not be the issue.

The next thing to check for is a bent derailleur hanger resulting in the derailleur cage not being parallel to the sprockets.

Another cause of shifting problems is the shift cable sticking in the housing due to corrosion or contamination, although this tends to cause 'delayed' upshifts (from larger to smaller sprockets) as the derailleur spring cannot pull the cable through the housing.

Given it was a new bike, with only a few months of use, the common cause is "cable stretch" or settling. Those twisted steel wires that run the length of your bike from the bars to the derailleurs will settle, meaning they get slightly longer, which is enough to de-tune the gears.

Almost all bikes have a "barrel adjuster" somewhere in the line. Its common to find them on the rear derailleur, though they can also be found sometimes on the downtube of road bikes, or perhaps even in the shifter on the bars.

To adjust for cable stretch, the outer housing also has to get longer. So unscrew the rear derailleur housing by half-a-turn and then test shifting with a short ride, or running through the gears with your bike's rear wheel off the ground.

The cable will stop settling after 100~500 km.

Most good bike shops (ie not boxmovers or chain stores) will offer a free 6 week service, where this sort of thing is fixed up. Its also a chance for them to upsell you on some accessories.

Just been given the apollo spiral as gears didnt work from day one and then owner was too embarrassed to go back as a novice female rider. I have had a look and the rear mech is intrinsically tighter than it should be and the front derailure has bent as it has caught the sprocket against the chain- It looked too low but on raising it I couldnt convince myself that unbending it would resolve the primary problem.... more likely the wrong radius of the arm for the gear ratios. Either way dont buy this bike.