Dear All,
Many thanks for your replies and the advice of experience contained in them all, I appreciate it. One question was regarding the disparity in sentencing. The US has the highest incarceration rates and the longest sentencing in the entire world, even accounting for those considered 3rd world countries (Economist Oct 2018).The US Federal system works almost 100% on plea deals and by such their conviction success rate is also almost 100%. This means that government prosecutors always go for the highest punishments they can inflict. Prosecutor is an elected office and high conviction rates mean public affirmation of their choice plus re-election when the time comes. So when it comes to charges, the prosecutor will demand a whole list of 'counts' that he/she thinks might stick, even loosely. Unfair but that's the system as the 10's of 1000's incarcerated will affirm. In demanding these counts, once a period of shock and horror has ensued,  an offer will be made that allows you some perceived relief that if you plead guilty to one or more counts, chosen by the prosecution, they will not oppose the others. In my case I had 2 counts, one with a mandatory 5 year minimum, the other with an advisory 5 year minimum but the judge is allowed discretion both up and down. Because of the relatively minor nature of my offence and please don't take this as an indication of my disregard for its nature, I was allowed to plead guilty to 'possession' but not 'reception'. You might argue that if you possess then you must have received and vis-versa but the Federal government sees it as 2 separate things, both with their own sentencing guidelines. Reception carries the mandatory sentence so I opted to plead guilty to the lesser charge and hoped (on advice) the judge would show leniency. That happened and hence 42 months. I know by UK standards, that seems harsh but as you've read in other posts, that was relatively light by comparison. Incidentally, those choosing to plead not guilty in a Federal court generally receive maximum sentences beyond the guidelines, hence the desire to choose the lesser options, regardless of circumstance or mitigation. I experienced that with many of my fellow inmates, regardless of crime.
One major difference I've experienced between incarceration in the US and also UK is that in the US, sex offenders are placed in general population unless the crime demands a separate location. Here in the UK, you are separated from general population immediately, even in prison transport which only adds to the hysteria and malice directed at this type of offence. Ignorance reigns supreme and is only enhanced by prison staff themselves along with the baying of politicians and press stirring things up for readership and public perception goals. While prison in the US was very much as portrayed on tv, so long as you make efforts to get along and avoid trouble, you'll be okay. Facilities for inmates there are far less than in UK prisons. Here in the UK you're instantly branded as a pervert or 'nonce' (I'd never heard that word until arrival here). Images of witch trials, ducking stools and ponds surrounded by ruff necked individuals in pointed hats springs to mind.
Anyway, onto my situation and it seems my addition to the register is almost certain but the question as to how long, is now my focus. As one contributor put it, the system is workable within means and that's where I'm at. Thanks to all again and good luck.

Just as an aside to this. I was reading about some of the US state laws governing RSO recently. Not all of these made it to the statute book, some were just proposals. Nevertheless, it's a reminder that, behind the perfect white smiles and 'Have a Nice Day' rhetoric, there are some really cruel and vindictive people over there.

RSO receive a visit from Probation every Halloween. They are not allowed to take part in Halloween activities, including driving their own kids to and from Halloween events. (Nevada)

A proposal that RSO be forced to have a pink or green licence plate on their cars, so they could be identified (Ohio). Echoing the pink triangle and yellow star from Nazi Germany.

Marking the driving licences of all RSOs with a special RSO stamp.

RSOs forced to display details of their offence on the side of their house.

RSO forced to wear a T-shirt detailing their offences. (Florida)

Local residents can put their email address on a list, to receive a message each time an RSO moves into their area.

Terrifying. I wouldn't want to visit the USA ever again, even I was allowed to.