Fiesta Gear Issue

[MrAdaam]

Copied from GG - Hopefully get a more 'Ford' response in here. TIA.

Now, I know that a '51 plate Fiesta is not the most PH of cars but I would like some advice on troubleshooting my gear issue. Driving to work this morning I had not problems changing gear, as smooth as usual, until I went onto accelerate onto a NSL road. I accelerated in second to around 40-50mph and then went to change up into 4th which became a massive problem as it seemed it had just been blocked off completely. I went back into second and rapidly slowed down.
I then carried on the journey after forcing it into gear each time I needed to change. It wouldn't cooperate at all and ever gear change was a struggle.

Where should I start looking? Do you need any more info? I'm not the most technically minded but I will have a go at some stuff.

[DownUnder.]

Possibly gearbox oils had it or selector, the gearboxes on these are pretty much bullet proof ib5s.
Best advice I can offer you is sign up on ukfiestanetwork and ask there, a lot of very knowledgable people who know the ins and outs of these zetec se/endura engines