Sime Gas boiler - acting up...any advice please?

It sounds like the system is low on water, or the circulating pump is not running. The bubbling noise is probably water boiling in the heat exchanger. It may be tripping out the high temp limit safety device requiring manual reset. I'd strongly recommend that you stop using it until this is resolved as overheating the boiler can cause expensive damage to boiler components. The boiler safety devices also need to be checked as the boiler should not attempt to fire under these conditions. Depending on the boiler either a low water pressure switch or a flow switch (or both) should inhibit the boiler ignition sequence. However I have seens many installations where the boiler low pressure switch has been disabled, paticularly when the heating system is filled from a roof tank via a check valve, rather than from the mains via a filling loop.

You can find a qualified gas installer who will look at your heating system here

The filling loop has a quarter turn black knob on it with a silver flexible hose and a non-return valve on it (some have what looks like a rad valve head on them). If it's not near the boiler it will be in the hot press. If you can not see it look in your hot press and look for a 1/2" pipe coming from the ceiling down to and connecting into the heating coil on your cylinder, this will be the top-up feed coming from the attic either conected to the mains or from the tank. There should be a valve on this.

You should look at why the system dropped in the first place. It's possible the safety valve on the boiler is passing and needs to be replaced.