Greenstar Heatslave producing very limited supply of hot water

You have no need to remove any Drayton cover to see the manual lever move on the diverter. Terry's post above gives you a bit more insight. With the best will in the world, I don't think you have the ability to effectively test your boiler, and would recommend you employ a suitably qualified/experienced technician.
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Follow the points above and you may solve it yourself. If the OP has not replied, then I doubt if he is reading this.
I suspect the solution was to employ a suitably experienced technician.
Actually, it didn’t ever get fixed. I’ve just been putting up with showers ending early, going tepid. I rush when using it, but still don’t get enough hot water. I thought I’m probably going to have to buy a new one. The service company ran out of ideas after checking the obvious stuff and sent someone out to ‘deal’ with me - his answer to every question I raised was “It’s all about variables, sir” I gave up at that point.

@jocie140 - did you solve yours?
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@Marcodn just thought I would reply as seen you have posed recently. We have had the same issue with this exact new combi being put in. Worcester Bosch have attended and explained the issue to us. The boiler itself is actually working as it should. It starts off super hot but it only holds so much and then as it goes on dependant on how cold the water going in is then it cannot heat it fast enough in order to have a shower that dosent end up lukewarm at the end. Same with the bath, we have been filling it up with only the hot tap as by the end of it the water is cold. Equally the recovery time is between 10 to 20 minutes for the hot water. Hence the reason we have been unable to have consecutive showers too.

Remedy for the situation is that we are having an unvented hot water cylinder being out back in to run the hot water upstairs in the airing cupboard, (somewhat irritating as they took out the previous cylinder which was only 18 months old when this new boiler was fitted) therefore being able to have consecutive showers/baths without running out of hot water. Downstairs will run on the combi as normal so heat on demand.
I think oil fired boilers have a primary heat store so when depleted might still expect the boiler to operate like one without one, ie HW being drawn off without any charging of the heat store. So, assuming store exhausted and assuming a 25kw boiler with 10C mains then HW temperatures of 46C at 10LPM, 34C at 15LPM and 28C at 20LPM. One way or the other the boiler should fire continuously while drawing off water at anything > 10LPM.
Example of Grant below.

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