That you have a Head Office suggests that yours is a multi site activity.

From the information given you are using consultants as your 'competent person'.

Some consultants will assist in the production of RAs and SSOW, but that cannot be carried out in isolation and must involve people doing the job to carry any credence. It will depend on the contract that is drawn up.

Welders and spray shop implies a need for Occupational Health monitoring, is the management of that element to be included?

Visit periods depends on the current state of your safety system and the upkeep required, lots of days initial set up, fewer days once established. Also ties in with the risk level on site and the risk appetite of the management.

It is likely to benefit the site on a day-to-day basis to have at least one person with a level 3 qualification, NEBOSH or NCRQ or equiavelent. As a minimum for information Managing Safely or an equivalent.

As to the rate, it depend what your risk level is and the qualification and expertise of your selected consultant. They can vary from low hundreds to a couple of thousand for higher risk areas, you pay your money and take your chance. Remembering that if they hold IOSH membership they must adhere to the code of practice on ability and validity of advice supplied.

Simply using a consultant does not absolve the company of holding the responsibility, in the long term it may be worthwhile training and developing someone in house.