Hi, for my old telly I bought a longer power lead, much like this one;

The new telly has a separate power brick with a 3 pin "cloverleaf" connector on it. Can I use something like this to use the old longer lead with the new TV's brick?

As the long lead can plug into the connector either way up, am I likely to cause something to go pop?
Depends if the new tv is double insulated or uses the earth wire.
If the former happy days, if the latter no no no!
Your new telly has an earth connector and your old cable doesn't. Now your new telly probably doesn't need earthing as it is likely to be double insulated but probably and certainly are not the same thing and if it did turn out it needed earthing you might well become that earth should something go wrong.
Why not just use a standard 3pin extension if you're happy enough with lead to lead?
It not the TV going pop that's the problem.nits you going pop.
if its got a seperate inline power brick, its probably stepping down voltage to 24 or 12v.
if you plug the straight 240v line in then it will go pop...
edit: which bit are you plugging into the TV?
which bit are you plugging into the TV?
I've shown the current connections and planned connections below, with my intended changes in bold.
Current connection;
Wall Socket -> Ugly extension lead -> Supplied short power lead -> Power brick -> Telly
Planned connection;
Wall Socket -> Longer power lead with wrong connector -> Adaptor from wrong connector to right connector (shown above) -> Power brick -> Telly
Or just buy a longer cloverleaf cable.
eg https://www.kenable.co.uk/en/computer-cables-peripherals/power-cables/3116-power-cord-uk-plug-to-c5-clover-leaf-cloverleaf-lead-5m-cable-003116-5055383431168.html
Or just buy a longer cloverleaf cable.
I originally tried that, but I'm specifically looking for a white one, and longer white ones seemed hard to find, so getting a short adaptor (that would be hidden behind the telly) seemed like my best option (assuming I don't get fried).
Just so we know will you be wanting a white coffin?
What @avdave2 said. You'll probably be OK.
The cloverleaf and figure-of-8 cables have the same power rating (2.5A) so you're OK on that score. But as you know the cloverleaf a) is also earthed and b) polarised. The million dollar question is, does this matter?
An appliance like a TV probably doesn't need an earth, it's encased in plastic for a start and if your home wiring is anything approaching modern then you'll be protected by an RCD or something similar which will trip near-instantaneously if there's a sudden anomaly in the current flow (because it's arced through a small child or something).
Similar with reversing the polarity (of the neutron flow). The power supply probably doesn't care (it's AC after all), just look at international travel adapters. Getting live / neutral wiring the right way round is critical in the mains wall socket because if it's wrong and the fuse in the plug goes pop then it won't isolate the live circuit; but transposing them after the fuse shouldn't make a difference safety wise that I can immediately see.
I've got to wonder though, with so many probables and 230V kicking about, if your primary concern is simply one of aesthetics then in lieu of chasing it into the wall / floor / shelf properly why don't you just install ducting? You can get self-adhesive stuff that's barely wider than a mains cable and will look a buttload better than daisy-chaining fifteen different cable types together.