There is an ongoing debate about ironing to 'rejuvenate' the DWR finish. The care label on a jacket says it's possible while the fabric manufacturers website says preferably not, various blogs have a number of differing opinions ranging from yes, to using a hair dryer, the inside of a car on a hot day (!), draping over a radiator..........So, does the application of heat make a blind bit of difference ?

Yes - heat treatment can provide some advantages in helping the treatment bond better to the fabirc and therefore last longer. It may not rejeuvenate an old treatment and definitely will not magically make it like new it if there is insufficient DWR coating on it in the first place.

Fabric manufacturers may have different instructions on their site, this may not necessarily refer to the exact [version of] fabric that your garment is made of, the best advice is therefore to stick to what the care label says.

What's the official line ?

If the care label says its okay then you can within whatever limits are prescribed for that particular garment. Generally, any garment suitable for tumble drying will be okay. Beware of overheating though as that can cause problems with delaminating some breathable coatings. I would be wary of using a hairdrier because applying an even heat will be almost impossible, a cool tumble drier will be more effective. 
 
If the third party product you are using says you should ( NIkwax / Grangers etc) but the care label says otherwise, then DO NOT. The care label is king and failure to follow it will void the warranty. Only chose products that the garment manufacturer recommends and that suit the properties and care instructions of the garment to be treated.
 
If in any doubt, contact the garment manufacturer and test on a small area that is not critical.


According to many blog contributors after market wash in proofers abrade so quickly that it's hardly worth doing............

After-market treatments are least effective when used on abraded, dirty or previously untreated fabrics but provided that they are used fairly regularly and prior to the original DWR wearing off, will help to maintain the garment for considerably longer and reduce the amount of abrasion damage by preventing dirt particles entering the fabric which then causes irreversable damage. ie: maintenance is better than cure !
 
I've just washed mine in Techwash and am thinking of a light hairdrying.

Techwash is NOT a proofer - it is a pure soap that will not degrade the existing surface treatment but will remove body oil and dirt deposits and  prepares the garment for application of a DWR treatment. Nikwax offers TX Direct in wash-in and spray-on form to replenish the DWR and is recommended by Montane and Rab amongst many others.