It could be physiological!

That is; the plant is rebeling with its new surroundings.

It might just be needing a bit of time to settle in to its new environment.

My thoughts are based on the fact that you say it is " just a couple of leaves" what are the remainder of the leaves like?

Is it still in the same soil as it was when you purchased it?

Thanks Teegee for replying, Yes it is still in the same pot I thought I would pot in a larger one when May is out, the rest of the leaves are ok and It also has a couple of buds and flowers on it although the flowers have been dropping there petals, I have it standing by the patio doors so loads of light.

Quote; standing by the patio doors

Is it in a draught, particular an icy draught!

Avoid overpotting!

Remove it from the pot and check the root ball to see if it is pot bound.

If it does need re-potting ( which is usually every two to three years) only go up one pot size at a time.

Watch the pH they are best on the slightly acid side say 6.0-6.5

To help to maintain this pH try and water with rainwater, particularly in hard water areas.

Thank you TG for your advice I will test the pH and water with rain water. Many thanks.

Looking at the picture, it looks like what we in the nursery trade call "truck blight". It is caused by the crushing and abrasion of the leaves during shipping, and also sometimes by the violent temperature changes involved. It looks like it might be an old leaf left from when it was shipped to the garden center.
Overfeeding would have quite different symptoms. I can't seem to find anything about "Bio tropical feed", though. Do you have a more exact name for the product you used?
TG does have a point about drafts, too.

Hi Tugbrethil, thinking about what you say "truck damage" makes sense as the orange plant that stands next to it is fine.

The plant food is actually called "Baby Bio Citrus".

Should I cut off the offending leaves as it is only a couple of them, I have also noticed this morning that tiny new shoots at the top are forming and look healthy.

Thank you

Yes, I would just take off the damaged leaves...and the new shoots are a very good sign! : )