You’re half way there really. Get yourself a large syringe like @LMSC did and use it (with a bit of tube)to suck all the water out of the boiler through the temp probe hole. Then inject descaler solution, until it’s full.
Make up 1 litre of citric acid solution by adding 50g of citric acid powder to 1 litre of hot water.
Leave it over night, then suck all the descaler out….. Put the probe back, allow the boiler to autofill. Heat the boiler up, switch off machine, drain thru hot tap, Allow to refill and reheat, taste water, if it’s OK you’re done, if you can taste citric acid…repeat the rinse.