In simple terms they are "fixing" the applicable regulation at the point in time of withdrawal so that if the EC subsequently amends the regulation for the remaining member states the UK enactment would no longer adapt or alter in accordance with such issued changes (often termed delegated regulations).

Given most EC/EU directives that were written in to law as UK Satutory Instruments e.g. CHiP have over the decades been superseeded by EC/EU regulation e.g. REACH/CLP there will likely be a significant number of these laid before parliament

This is because of something called the “Sovereignty of Parliament”, which essentially means that under UK law the Houses of Parliament can through an Act of Parliament make up any law they want with no restrictions even EU law. The European Communities Act which is what is repealed here says in essence that under the Act EU law including EU relations take precedent  over UK but only as long as the UK Parliament explicitly says so.  As soon as it says that the EU laws no longer apply, they no longer apply. To enable us to continue Parliament has had to pass a law that says for now all EU applies as if it was UK law. What happens next is the interesting bit such as what will be the role of the European Court of Justice: everybody knows that it is not just the law as it is written that is relevant but how it is interpreted by the courts. In the case of EU law that is the ECJ but we will not be under that court’s jurisdiction anymore so we could find ourselves in a situation where our courts interpret the law differently for the EU courts which means that in effect it will be a different law which then begs the question what is the point of retaining EU laws if we are going to apply them differently?

Eventually Her Majesty’s Government will try to get rid of those laws that they regard as “red tape” and “a Burden on Business” but what laws will those be?   

Now I want to cry because I felt it necessary to actually read the European Union (Withdrawal) Act 2018.

It states that EU Regulations (among other EU laws) are 'retained' and remain UK law in the form they have at withdrawal (that is, not subject to any future amendments by the EU).  It goes on to specify how 'retained' laws may be amended. (That is the type of amendment that was puzzling me.)

But, there is no actual copy to be made of EU laws at the time of withdrawal, they just have to be imagined as part of UK law as if they had been copied into UK law.  The amendments to them are amendments to imaginary copies so we just have to imagine the 'as amended' version,  which is not going to be written down anywhere in a form that you could actually consult it.  You may not even profit from consulting the EU legislation directly as it may have been amended by the EU and these amendments don't count.

Have I got this right?  Without wishing to make political comment, it feels like a terrible mess.  Really struggling to get my head round it.

I am a simple-minded person who likes to have the statute law of the land laid down in writing so anyone can see what it is instead of having to cross-reference to potentially superseded EU laws and then mentally apply UK amendments to them.  My bafflement now is that the government thought this was a good way of going about it.  But then, I'm baffled by much of the way the government has handled the issue right from the start.  It's not how I would have done it ...