Most times I have found that only gas engineers provide power flushing service as by requirement if they fit new biiler they have to flush system as a part of commissioning.
Power flushing equipment is very expensive so a plumber who's jobs are such as bathrooms kitchens general plumbing maintenance do not buy those machines as it will take ages to pay them off.
I am a plumber who has this equipment but I would suggest to post a job for gas or heating engineer.
I hope this will help
Both plumbers and heating engineers should be able to carry on a power flush, providing they have the equipment. Chemicals are available at almost any plumbing supplier.
However before deciding to go for the power flush you need some specialist advice, power flush is not like magic and all problems solved. Also consider installing a descaler or water softener to prevent the need for a power flush in the future.
I reckon your best option is to call in someone qualified and experienced for assessing the problem.
Hope this helps.
Usually heating engineers but before you think about it there are a few things to consider, if the system is micro bore you you create more problems doing a power flush, if there are cold spots on the rads and they are old try looking at replacing them rather than flushing, might save you money