We don't know 'where it's at' do we? They are not giving a running commentary. The fact they have been given yet more money, to me, shows they are following a lead.
Let's assume that this 'lead', this one remaining lead is as obscure, compelling, elusive and fugacious as possible. What could it possibly be? An elusive person of interest? DNA or other forensic material that is awaiting new, or emerging technology to analyse or unravel, or some other reason for a forensic delay, such as attempting to gain a sample or match? Waiting for a person of interest to make a mistake? Waiting for 'just cause' to execute a warrant? Scouring a large land mass for remnants of evidence, something painstaking and meticulous that requires a team of specialists, like sifting through big data? But the numbers don't support any of those hypotheses, apart from one. £300k for another year seems like a large sum of money, but in policing terms it's actually not much at all. According to fullfact.org a police officer on the lowest pay point would cost £28,600 in London and £25,400 across the rest of England and Wales, including their pay, tax and pensions contributions. After four years’ service, that will usually rise to £35,500 and £32,300 respectively. Let's assume £35k per operative for the year, which is £140,000 for 4 officers. But this is just the standard salary rate - internally they will be assigned a cost code and the 'charge out rate' is often way higher inter-departmentally. Factor in a part-time DI or DCI, then the budget is all but spent -
https://www.metfriendly.org.uk/services/police-finance-information/police-pay/ - level 1 inspector = £52,722 + £2,373 / 2 = £27,547. So it would appear just simple bums on seats equates to most of the £300k - without factoring in resources, internal and external - cars and fuel, printing, civvy admin, welfare, assistance with IT / analysis, etc, etc - all of which is internally charegable.
What tactic could the investigators be deploying that took so long and required a further injection of funding? What convinced the Home Office that this was worth the continued and costly pursuit?
Let's crunch the possibilities. What are the strings to their bow - all of them? Pursuing an elusive person - how elusive would you need to be? Lord Lucan elusive - you have eye witness accounts of your quarry and are actively pursuing them, like hunting for Martin Bormann in remote backwaters in Argentina. Problem is, there's been no appeal for information - the public is not being asked to get involved. You don't want your target to go further underground, but you need more eyes and ears - it's a trade off. Besides the money isn't there to chase a phantom around the globe.
Waiting for forensics technology to catch up? Well haven't NSY been offered cutting edge, new analysis of existing data by Dr Mark Perlin, depending on who you believe. The refusal / non-response should not be construed as 'not required' however, as this may also be fraught with legal difficulties, or the samples may not even exist, even if Dr Perlin's credentials and techniques can be verified.
How about digging great swathes of Portugal up? The press would be on to that and the resources are mis-matched - it's not happening.
Detailed data analysis - again, those resources attract costs and they are finite - there's a queue and priorities, such as heatmapping knife crime, etc.
What else? Surveillance. What does surveillance cost if you've already paid for the core team of bums on seats? Not much more. It's business as usual, isn't it?
And that, if anything, over and above simply keeping the lights on and the phones answered, seems most plausible. £5k a week is nothing. A routine murder enquiry will burn that in a day (assume 10 officers / support at Met rates as per the cite above).
Listening to covert surveillance equipment in a cupboard in London on shifts is just about business as usual. It's costly and time consuming, probably just costly and time consuming to require further, ongoing, indefinite support, but the 'target' would have to be pretty compelling i.e. there's other evidence, but not enough to convince a wary Home Office and a circumspect and nervous CPS.