Mistakes on tax form

If I have made mistakes on the tax form but haven’t paid yet, and haven’t had everything finalised, do you think I will be able to explain properly to the tax office and sort it out. I will explain the reasons to them properly. Or would it get me into trouble?

The reason I made mistakes was basically because my old employer would not give me any details, dates, numbers, with holding statements, etc etc. So I pretty much guessed it all.

This is really a difficult question to answer without all the details. If, as you say, you were estimating because of a recalitrant employer, it might be best to come clean if you now have all the correct information. It sounds like your employer is in violation at your peril. The tax office can assist with this employer. If you now have all the correct information, explain that you would like to amend the return. On the other hand, if you have been less than forthright and assuming the employer doesn’t correct his omissions, then maybe silence is golden.
If it were me, I would go and explain but . . . be careful.