There is no reason it shouldn't be provided you are happy with removing the panels and replacing them to gain access, all the parts are available if you don't fancy the ash work yourself.
Bob has passed on now but Roger is most helpful and knowledgable and will give you good advise. I bought my car from them a couple of years ago they are one of the older established companies.
2009 4/4 Sport Henrietta 1999 Indigo Blue +8 2009 4/4 Sport Green prev 1993 Con Green +8 prev
To do the job properly, I would think you'd need a specialist, and I suspect the resultant bill would not be small - a lot of the tub would have to come apart to get to it.
If it were me and I found myself at a pecuniary disadvantage, I'd probably use liquid epoxy wood repair to stabilise and harden the existing wood, drill and inject a few boron rods to kill off any rot and prevent any new occurrence, screw a strengthening piece below or behind it to restore the strength, and re-form the profile with epoxy to make it look neat again.
If they can use epoxy injection and boron rods to fix roof timbers in houses, it should be ok for a non-critical body frame component, I would have thought.
Tim H. 1986 4/4 VVTi Sport, 2002 LR Defender, 2022 Mini Cooper SE
Looks a bit like dry rot to me. If you have the hand skills plus tools, it's a straight forward job. The issue will be whether the rot has affected other areas making it a much bigger job.
If its spongey and damp them it's wet rot and just needs the soft areas replacing and surrounding areas hardened with wood hardener
If it's dry rot it goes powdery and will be dryer to the touch - this is caused by a fungal infection eating away and is a bit more serious as the fungus spreads
2008 XXVII Platform, Bugatti Blue Roadster 4 Seater
Here is a scan of an old Harpers catalogue and might be of help to you, it's about 11 years old so the prices will be irrelevant but the part numbers should be consistent.
No fancy showroom or salesman here but their wealth of knowledge, enthusiasm and love of the brand is infectious.
2009 4/4 Sport Henrietta 1999 Indigo Blue +8 2009 4/4 Sport Green prev 1993 Con Green +8 prev
I have used Tim Harris's approach for years. It is called "CPES" Clear Penetrating Epoxy Sealer". Not sure what they call it in the UK. But a Marine Chandler will know about this stuff. But as somebody on TalkMorgan pointed out this looks like the "Tip Of The Iceberg". I have repaired structural damage that was worse than the picture. But I am thrifty (cheap) and I try to catch this stuff before it gets to be major. I have used Epoxy Putty to rebuild pieces of wood and painted them so one really would not know I had used this approach. You will hear over and over, "If You Are Going To Do It Do It Right". Yea and have money to burn also.
If this piece is just screwed in and accessible than it would be crazy to not put a new timber in. In any event You would have to remove the back deck. On My +8 Bitsa it is easy. But not sure about a later model.
I would expect wet rot not dry 
cut it out immediately before it spreads 
