[As stated your first port of call should be the manufacturer's washing instructions](https://www.regatta.com/garment-care/)

I'm not famailiar with this fabric. [Quick google of IsoTex basically shows it's Regatta waterproof fabric](https://www.regatta.com/fabrics/). Not much details on how it's made, etc. but it does state it has a DWR

> The DwR outer fabric reinforces the waterproof quality and helps the
> water to run off whilst adding to the garments windproof performance.

If it's got a Hydrostatic Head of 5000 then it's not as waterproof as Goretex. Looking at the coats it's made from and the price point for this makes me think it's not got got a physical barrier like GoreTex (or if it does it's not as sophisticated).

Bearing all the above in mind you want a non-soap cleaner, I always use [NikWax Tech Wash](http://www.nikwax.com/en-gb/products/productdetail.php?productid=4). Regatta recomned their own brand, but well they would... I've always had good results with the NikWax one though. Don't use normal washing powder, this will reduce the effectiveness of the DWR.

I would definitely then apply a new DWR. From looking at similar fabrics and reading reviews a good application of DWR is pretty essential to this coat, without it, it isn't going to stay waterproof for long. 

I prefer a spray one one as opposed to a wash in one. It allows you to be more targetted to it's application, i.e. only spray it on the outside where it's supposed to go. 

Tumble drying will improve the DWR. DWR works by tiny microscopic spikes keeping the water from soaking in. Over time these spikes wear down. Tumble drying (heating) allows the chemicals to re-form these spikes. **Will it ruin your coat** What does the label say? If it says do not tumble dry don't. If it says you can, it will help and is worth doing.