I'm a year behind you on the experience front, and God knows should be doing alot more on CPD than I am!
What do you mean by design inspections? Do you mean inspecting properties/structural works and reporting on the results or do you mean carrying out design work or both?
As far as I know, as long as you're not misleading anybody as to your qualifications (i.e. you're not claiming to be officially affiliated to IStructE or ICE when you're not entitled to) then you can do pretty much anything you want. It's not like the US system where you have to be licensed.
The big thing is indemnity insurance. If you're not chartered nobody will insure you. If you make a bad recommendation/design and it costs somebody money, you could be personally liable (financially).
An area I know nothing about is building warrants/insurances. I would think that there might be a requirement to provide the credentials of a designer - someone else might know better.
I know there's a clause in my employment contract which forbids me from doing any work of the sort the company would normally do, without consulting one of the directors first. This is another thing to look out for.
Who's your mentor (FIstructE or MIStructE)? They could probably answer all this for you.
The major thing to think of is personal liability. If you design something, it falls down and kills someone, you could then end up in the dock on a manslaughter charge. Not being chartered would probably count against you big time, as you should know better and have your work checked.
That's the officaldom of it anyway.
Jobs on the side always have and always will go on. It's up to you to decide on the risk involved. Legally however, I don't think there's anything to stop you.
Right, I think I'll go and note this down as half an hour's CPD - discussing the legalities and responsibilities of the profession