My 'Sent' folder is corrupted and cannot be opened.

How do you determine it is corrupted? What prevents you from opening the Sent folder? How do you 'open' the Sent folder anyway?

As with all folders, the way to open is to click on it. When I do so, I get a blank screen. The title of the folder is also in grey italics, which not normal.

Please post your Troubleshooting Information. At the top right of the Thunderbird window, click the menu button , then select Help > Troubleshooting Information. Press the "Copy text to clipboard button" and paste the information into your reply.

Application Basics

   Name: Thunderbird
   Version: 45.8.0
   User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:45.0) Gecko/20100101 Thunderbird/45.8.0
   Profile Folder: Show in Finder
   Application Build ID: 20170305125302
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts
   account1:
     INCOMING: account1, , (imap) mail.electro-pen.co.uk:143, plain, passwordCleartext
     OUTGOING: , smtp.electro-pen.co.uk:587, plain, passwordCleartext, true

   account2:
     INCOMING: account2, , (none) Local Folders, plain, passwordCleartext

 Crash Reports
   http://crash-stats.mozilla.com/report/index/bp-2461ecf3-c9bb-4d12-9882-3e9f00170914 (15/09/2017)

 Extensions
   Lightning, 4.7.8, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Important Modified Preferences

   Name: Value

     browser.cache.disk.capacity: 358400
     browser.cache.disk.filesystem_reported: 1
     browser.cache.disk.smart_size_cached_value: 358400
     browser.cache.disk.smart_size.first_run: false
     browser.cache.disk.smart_size.use_old_max: false
     extensions.lastAppVersion: 45.8.0
     gfx.blacklist.direct2d: 3
     gfx.blacklist.layers.direct3d9: 3
     gfx.blacklist.webgl.msaa: 4
     gfx.crash-guard.glcontext.appVersion: 45.8.0
     gfx.crash-guard.glcontext.deviceID: 0x71c5
     gfx.crash-guard.status.glcontext: 2
     layers.acceleration.disabled: false
     mail.openMessageBehavior.version: 1
     mail.spotlight.firstRunDone: true
     mailnews.database.global.datastore.id: 9bc64ffc-6e7c-e343-bbbc-77cc55cbafc
     mailnews.database.global.views.conversation.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"3"},"attachmentCol":{"visible":false…
     mailnews.database.global.views.global.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"3"},"attachmentCol":{"visible":false…
     network.cookie.lifetimePolicy: 2
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1518545021
     places.history.expiration.transient_current_max_pages: 80531
     plugin.importedState: true
     privacy.donottrackheader.enabled: true

 Graphics

     Vendor ID: 0x1002
     Device ID: 0x71c5
     WebGL Renderer: ATI Technologies Inc. -- ATI Radeon X1600 OpenGL Engine -- 2.1 ATI-7.4.1
     GPU Accelerated Windows: 0. Blocked for your graphics card because of unresolved driver issues.

     AzureCanvasBackend: skia
     AzureSkiaAccelerated: 0
     AzureFallbackCanvasBackend: none
     AzureContentBackend: quartz

 JavaScript

 Incremental GC: 1

 Accessibility

   Activated: 0
   Prevent Accessibility: 0

 Library Versions

     Expected minimum version
     Version in use

     NSPR
     4.12
     4.12

     NSS
     3.21.3 Basic ECC
     3.21.3 Basic ECC

     NSS Util
     3.21.3
     3.21.3

     NSS SSL
     3.21.3 Basic ECC
     3.21.3 Basic ECC

     NSS S/MIME
     3.21.3 Basic ECC
     3.21.3 Basic ECC

This is an IMAP account. Can you login to your account via webmail, and check that all messages are on the server?

INCOMING: account1, , (imap) mail.electro-pen.co.uk:143,

Is it intentionally not to use an encrypted connection for IMAP?

Do you have 'Synchronization' enabled for the account in Thunderbird? https://support.mozilla.org/kb/imap-synchronization

If the Repair Folder step doesn't help, you could rebuild the Sent folder according to these instructions. Basically, you copy existing messages to another folder, delete the Sent mbox and msf files from the profile folder, then restart TB to automatically create a new Sent folder. With an IMAP account, the sent messages should then download from the IMAP server. To open the profile folder, Help/Troubleshooting Information, click Open Folder; close TB before making any changes.

As I notice your domain is hosted by Fasthosts, you may want to consider setting up your accounts with secure settings.