Servicing my car?

I recently got a different 4x4, 2.5 diesel. it looks a bit sluggish, is that because I did have a four litre V8 engine in the last one?
OR can it do with a tune?
I dropped the oil out and changed that, new air filter/oil filter and fuel filter. What else could help 'perk' it up?if anything.
Diesel so got no plugs.

Well, there is a hell of a difference between a 2.5 and a 4.0 V8. you will never get the performance the V8 was able of, not without some serious tweaking & tuning which can make your new engine somewhat fragile and intractable. What you'll get is considerably better fuel intake and longevity

You could look at cleaning or replacing the injectors, checking the injection timing etc., but if it is running well I'd be inclined to leave it alone. Maybe you could attempt some injector cleaner in the fuel, it may or may not make a difference. And there is the trade trick of chucking injector cleaner in it and taking it for a hard, full revs blast up the road for some miles. Foot hard down in 3rd does the trick. That works wonders to clear out the cobwebs.

red ex diesel treatment from most petrol stations

Diesels are not noted for off the line power. they're pullers with lots torque.

My guess is you're noticing the difference between a V eight gas and going down to a smaller diesel.

There are after market "boosters" for some diesels, but most will void any warranty you may have.

oil change and new filter does not really get you anywhere close to your old 4L V8. you may be able to get new intake, headers, test pipe, catback exhaust, pullies, lighter clutch plate/flywheel, supercharger, brighten your car.

Your new truck has an engine that's about 1/3rd the size of the old one.

Although it woudln't hurt to have a wrench check it out I am thinking it is running just good.

Have the pump pressure checked out and be sure any hoses are nice and secure.
Make sure it 'rolls' freely when the brakes are all off.
Other than that, as you there's little to check on a diesel.
It will seem slow after your V8 and you'll need your foot to the floor to get anywhere through the gears with it.

There is little you may be able to do about a deisel seeming sluggish in comparison to a gas engine, it's their nature.
Just accept that it'll not rev as quickly as a gas engine, but will do so for 250,000 miles or more.
I once encountered a stationary deisel that had been running at 1800 RPM for 60 YEARS.

unfortunately there will be a vast difference in performance, not lots you may be able to do, give it a month or two and you wont notice it so much.