Compost

If you.are putting weeds and grass clippings in, and they are moist (not bone dry), they should start rotting almost immediately.

If you want to speed things up, add some granular fertilizer, maybe a half pound or less, to your dalek. If you think the compost seems dry, sprinkle with some water, you want moist but not soggy. The fertilizer and water will warm up your compost and speed decomposition.

As soon as the weeds are pulled up and the grass is cut they go straight in the dalek, along with a couple of handfuls of Garotta

I try and cut the grass every two weeks, but my dalek is full now, I know its not going to rot away to any extent in two weeks, so would keeping it in a black bacg for a couple of months be okay ?
I dont want to end up with a bag of green slime

I don't think keeping it in a bag will work.
Air is the key. Keep air out (bag) & you get silage, not compost.
You have too much green waste for your dalek & the best thing would be another one & some more brown stuff eg cardboard to mix in with the grass. Fill the new one for a while until the first one rots down. Three is even better - you can combine the first two into one, once they halve in size.
Be careful , composting is a slippery slope....

Plastic bags full of anaerobically rotted grass sounds like a bad idea ( there will be a lot of bad odor when you try to switch it from anaerobic to aerobic composting).

If you are out of space in your dalek, there are 2 solutions, either set up another composted to accommodate more yard waste, or leave the grass clippings on the lawn and mulch them in ( mulching lawn mower blade).

A third option for some is to send them to a municipal compost facility that accepts yard waste (accumulate in paper leaf and limb bags, or possibly cardboard, depending on what's accepted).

Okay, black bags arent such a good idea

What about just piling the cuttings, weeds and grass in a heap and turn them over once a week ?

I could get another dalek, but I was thinking of that September/October when I will have a bit mor room and probably a lot more stuff to go in it

I use a Flymo hover lawn mower and rake up the cuttings

A compost heap for cuttings/weeds is a good "old school" solution.    You don't mention food scraps going on that heap, you are wise to leave those out, if there's even a remote chance of rats around.

For the grass cuttings, mulching on the lawn is still a good practice, as that returns nitrogen to the lawn.

I'm going to use cut and dried grass clippings on a couple beds as a mulch layer. That's another way to use them.