Sudden increase of electricity usage?

Where are you seeing that ? Do you have half hourly readings from a smart meter ? Can you post a pic ?

Sure 

I think you might need to try isolating circuits @Wimbo as there could possibly be the start of a short on your wiring. 
On your fuse board / distribution board where the fuses or trips are located, when you can, switch one off for a while (say an hour if possible) and look to see if that reduces the reported usage. You need to do it one at a time so that you can see which one has the higher level. 
It looks like you might have something that is causing a 300 w draw which could be an appliance or just wiring beginning to short. 
It’s important to find so that your house is safe as well as taking your usage back to where you expect. I presume you had the low level (it is very low) on several days before 8th

Immersion heater?

650W is too little for an immersion heater (except perhaps for an industrial scale tropical fish tank). It’s more like a warming drawer that the cat switched on. I can’t think of any other common bit of household equipment with a rating like that, other than ones you’d notice if they were operating continuously.  
 

@BPLightlog Sorry if I’m mistaken, but those are 30 minute consumption figures. The power draw in kW is 2x the kWh value shown. 

Something drawing 650 W would be noticeably hot wouldn’t it ? We have an electrically heated towel rail which goes up to that. Excellent example of a smart meter showing unexpected energy use !!

Thanks all, I’ve started to switch off some of the appliances on the fuse box and no consumption seemed to change yesterday, consumption was the same. I’m starting to get very concerned… it’s a 2.5 new build, we’ve had lots of problems with the build, any other recommendations to identify the problem would be much appreciated! There is a water pump on the lower ground floor, which takes the water out of the patios up and out to the waste water, any idea if it could be a problem with that?

Sounds very possible. Can you tell if it’s running constantly? Are you able to switch it off and then check the meter? 
Just scanning through a description of a ‘dirty water pump’ - could there be a float switch which has failed so it’s not switching off?

I have turned my whole fuse box off and this isn’t effecting the usage at all, it’s still showing as a constant high electricity consumption for all day