Hello again, thank you for your reply.
From the description you've given this is down to the clutch slipping. The clutch consists of a clutch cover, a center plate and a bearing/cylinder assembly. The center plate is clamped between the clutch cover and the flywheel of the engine, it has a hole in the middle that is located on the gearbox input shaft, as the flywheel turns so does the center plate clamped between the flywheel and the clutch cover which turns the gearbox input shaft. When you press the clutch the bearing pushes against the clutch cover causing the cover to release pressure on the center plate allowing the flywheel to turn without turning the plate and so the gearbox.
When the clamping pressure on the center plate isn't tight enough the plate "slips" so not transmitting all the drive from the engine into the gearbox. The higher the load on the plate when it is slipping the more it will slip and the less drive is transmitted to the gearbox. This explains why when you try to accelerate hard the engine speeds up but the acceleration of the car is slow. If you accelerate gently then there isn't as much load on the center plate so it is less likely to slip.
As you say it is getting worse, if left long enough the clutch will slip too much so no drive is transmitted from the engine to the gearbox so the car won't drive.
To replace the clutch means removing the gearbox so it is a little costly as this is around 5 hours labour plus the parts, also this engine has a dual mass flywheel that is a little prone to failing so don't be surprised if when a garage does the clutch they tell you the car wants a flywheel as well. I would ring a few garages to obtain a quote as a lot of garages replace clutches for a fixed fee.
I hope this answers your question but if I can assist further or if you need clarification of any point then let me know and I'll reply as soon as I can.