If your car doesn't have the bearings at the base of the main springs, fit the Mulberry kit -- it will transform the heaviness of the steering. Much cheaper than fitting power steering!!!
Cheers
Actually they won't make a difference at parking speeds which I suspect may be the OPs main problem.
I have to do a lot of low speed manoeuvring on my +8 and it is hard work, I can manage it at the moment, but if in some time in the future it gives me problems than I would certainly consider one of the programmable electric ones and have it set up to give no assistance above (say) 15mph