I found I can import a gpx file as an xml file into Excel (I use Excel 2016) and the data is all in neat columns - Speed, timestamp, cadence etc. So now if there is bad data I can edit it -example my HRM picks up stray data showing bpm above 230.
My question is how do I re-save the file so I can import it back to Strava as a gpx file? Excel says I can't save as a xml file. Is there some plugin I need install?

You can certainly edit them in a text editor. I often export the previous day's commute and do a find and replace on the date if I forget to record a ride. It won't be tabulated as neatly as in Excel but if you use something like Notepad++ that has an understanding of XML syntax you may find it ok

Yes- I know the notepad trick. Trouble is my HRM data has a couple hundred bogus points I want to edit. In both notepad and word I cannot use find/replace probably because the data has too many symbols and Word thinks symbols have meanings in find/replace - here is a typical entry in the gpx file for HR: <gpxtpx:hr>116</gpxtpx:hr>

Thanks deapee - notepad++ will do it under search>replace
Highlight all the text which has the erroneous data. Tick "In Selection" and "regular Expression"
find: hr>… (each dot is represents a single number)
Replace with whatever figure you want (eg hr>120)
Hit "replace all" and note the number of replacements made.

there's a program out there called Free XML Editor (FoxE)... give it a whirl. I use it all the time to identify bad data in XML files (by bad, I mean non-compliant, won't display etc, not necessarily an unwanted number)

Free XML editor download

Have you tried exporting data as CSV (comma separated values)? Then rename to GPX. I have not looked in a bit but I think that's how excel saw your data. They are just pure text files.

Well, I guess it is in a kind of .xml. I use GPX track editor to clean up and divide tracks. Never tried changing data but you can delete sections and export selections. I normally use it for rock hounding trips and offroading.
Wife was after me before to "get going if you are going for a ride before it gets any hotter". Did 12 miles.