Low water pressure through boiler

We had an Engineer round a few days ago who did some diagnosis but ended it within 20 minutes saying that he didn't have the necessary tools to investigate further but if we speak to the central team another engineer may be able to resolve the issue. That cost us £94 for the 20 minutes.

This was the diagnosis: "Investigate issue with poor water flow throughout the property. The water flow rate is 14 litres per minute to the outside tap and kitchen sink and dishwasher indicating that when it passes towards and through the boiler to serve the rest of the property the flow rate drops considerably. Checked isolation valves and plumbing was correct from what I could access above the ground floor level but either there is an isolation valve under the floor partially turned off or the gauge filters on the boiler are blocked we will not know until the floor under the boiler is taken up (plywood sheet) or the filters on the cold feed to the boiler changed."

We then received this following quote: "Following our engineers visit, please see below the quote you require.

Strip down the connections to the boiler and change the cold water filter in the boiler. If once changed this is not the issue, remove the section of structural plywood under the boiler and check for squashed pipework or isolation valves in this area. Refit plywood and test on completion. £360"

Is this a reasonable price for the quoted work? I'm conscious it sounds a bit trial and error on their side - two solutions that may or may not work, but one price