D light blinking an consumes a lot of petrol?

The blinking light means there is at least one code stored for the transmission system. That would cause both issues. However you would need to use a scan tool to read the code(s). Then use the code(s) as a guide to discover the actual problem. It could be as simple as a sensor on the transmission or as bad as internal parts failure. Honda didn't include a way to read the codes without a tool though, there are some inexpensive code readers available these days but you need to be sure they work with Honda.

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Thank you Steve. I will run the diagnosis scan today.

Please, Let us know what you find.

The OBD II showed a P0752 code -Shift Solenoid A Stuck ON.

That would be an internal solenoid that has failed. The troubleshooting is to drain the transmission fluid and look for any metal in it. If you find metal, the transmission is tearing itself up and needs replacement. If there is no metal then you replace the transmission fluid, clear the code and see if it returns within a few miles. If it does then the next thing would be to replace the solenoid that is under the solenoid cover. Then see if it is repaired. Generally what I do with these is drain the fluid, strain it and look for metal. If there is none in the fluid then I pull the solenoid cover, replace the one in question and reassemble with new fluid and then go for a test drive. If there was no metal this was a 80% repair. the others required a new transmission.

I just visited the workshop today. I was told that the solution is to change the entire valve body. The reason being if any of the solenoids has a problem, then the standard solution they provide is changing the valve body instead of changing the solenoid at fault.

Not sure I would swap the entire thing for a single solenoid unless there were signs that the other solenoids were failing. Especially if you are trying to discover if the issue is just the solenoid and not an internal transmission failure. At this point it could be either and I would try to save some money. Pretty much the steps I would do is erase the code, does it come right back? Yes - drain the transmission fluid. Do you see metal? Yes - Plan on transmission replacement. No metal, remove the cover and change the solenoid out. Reinstall the cover, fill with fluid and take it for a drive, no codes and shifts OK? Yes - done BUT it may have reduced the life of the transmission if it was trying to keep the transmission in two gears at once. No doesn't shift correctly or still sets the code, plan on replacing the transmission. I might suggest visiting a second shop, see what they say, then you can determine which way you would rather go.