If someone was to change the Masthead Banner and Logo of their website would this in turn affect any online sales that is done through the website or lose a certain bit of visibility from the internet ….

It depends on what it is, if its a link or just a picture with the link as part of the coding.

Martins probably best placed to ask but IMO as long as its just a picture thats changing and not the code behind it then it should be ok  FFRR MY06 facelift With TDV8 Alloys Zeros/ATR's
Mantec Sump Guard, Rigid Load liner, MY10 BT upgrade.

Re jigging the company name to make it look a bit more modern and changing a couple of the main pictures on the front page updated so to speak Will …

As long as it's just the image then it should be ok. Are you doing it yourself or an IT guy off you? FFRR MY06 facelift With TDV8 Alloys Zeros/ATR's
Mantec Sump Guard, Rigid Load liner, MY10 BT upgrade.

It will be a company that does the hosting for the website that will be doing it

Just make sure that they agree to no more than 5 minutes downtime and that they fully test it.

If it's a managed service then all you can do is leave them to it and be polite if it fails  FFRR MY06 facelift With TDV8 Alloys Zeros/ATR's
Mantec Sump Guard, Rigid Load liner, MY10 BT upgrade.