OK a few options and things to check from free to most expensive (some mentioned above)...
* get a pair of molegrips onto the wastegate actuator arm, you should be able to push the arm toward the actuator, it'll be stiff because you're fighting against a 16spi spring. if you can't get it to move you'll need to remove the turbo heatshield, unclip the arm from the turbo and WD40 the wastegate arm and free it off
* clean the MAP sensor, it's on the inlet manifold
* clogged air filter - eyeball it
* delaminated tubo hoses, take them off and look inside them to see if they are collapsed, replace with silicon
* wastegate modulator failure - find this at the end of the small hoses connected to the turbo wastegate actuator, if this fails it can cause an overboost scenario where the ECU cuts fuel, I would NOT expect to see this happening at 1900 rpm though.
* clean or remove the EGR system, it could be stuck.
* disconnect the wiring from the MAF and see if performance improves. If it does, replace only with a VDO MAF - these are the OEM, should be about 65 quid
* 02 vehicles have a CAT in the downpipe, it's common for these to clog, if this happens performance is pants. Replace with a decat downpipe.
* failing fuel pump and/or fuel pressure regulator, bit harder to check for but you need to be seeing 4 bar of fuel pressure at the FPR - can only be checked by putting a pressure gauge into the hole that the fuel temp sensor sits in.
Getting it hooked up to diagnostics and someone eyeballing the data would potentially be a much faster route to diagnosis.